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[libvirt] Re: [Libvir] Reopening the old discussion about virDomainBlockPeek



Attached is an updated patch.  The changes are:

 - updated to latest CVS
 - run make check / syntax-check
 - remove virsh subcommand (as per Dan's suggestion - see below)
 - some more things that Dan pointed out - see below

I would like to add this to CVS because it is quite a pain tracking
CVS changes.  I know there's no remote support at the moment, but I
can add that later.

On Thu, Apr 24, 2008 at 03:02:11PM +0100, Daniel P. Berrange wrote:
> > Index: configure.in
> > ===================================================================
> > RCS file: /data/cvs/libvirt/configure.in,v
> > retrieving revision 1.139
> > diff -u -r1.139 configure.in
> > --- configure.in	8 Apr 2008 16:45:57 -0000	1.139
> > +++ configure.in	16 Apr 2008 15:18:07 -0000
> > @@ -60,6 +60,10 @@
> >  
> >  LIBVIRT_COMPILE_WARNINGS(maximum)
> >  
> > +dnl Support large files / 64 bit seek offsets.
> > +dnl Use --disable-largefile if you don't want this.
> > +AC_SYS_LARGEFILE
> > +
> 
> IIRC, this is redundant now - we already added it elsewhere in the configure
> script when we did the storage patches.

In the patch I removed the second AC_SYS_LARGEFILE and left the useful
comment.

> > -
> > +int                     virDomainBlockPeek (virDomainPtr dom,
> > +					    const char *path,
> > +					    long long offset,
> > +					    size_t size,
> > +					    void *buffer);
> 
> Should probably make offset be an  'unsigned long long' unless we have
> some semantics which want -ve numbers ?  Is 'char *' better or worse
> than 'void *' for the buffer arg ? 

I changed the offset to unsigned long long.  Left the buffer as void *.

> > +/* "domblkpeek" command
> > + */
> > +static vshCmdInfo info_domblkpeek[] = {
> > +    {"syntax", "domblkpeek <domain> <path> <offset> <size>"},
> > +    {"help", gettext_noop("peek at a domain block device")},
> > +    {"desc", gettext_noop("Peek into a domain block device.")},
> > +    {NULL,NULL}
> > +};
> > +
> > +static vshCmdOptDef opts_domblkpeek[] = {
> > +    {"domain", VSH_OT_DATA, VSH_OFLAG_REQ, gettext_noop("domain name, id or uuid")},
> > +    {"path", VSH_OT_DATA, VSH_OFLAG_REQ, gettext_noop("block device path")},
> > +    {"offset", VSH_OT_DATA, VSH_OFLAG_REQ, gettext_noop("start offset")},
> > +    {"size", VSH_OT_DATA, VSH_OFLAG_REQ, gettext_noop("size in bytes")},
> > +    {NULL, 0, 0, NULL}
> > +};
> 
> 
> I'm wondering if this is perhaps one of the few APIs we should /not/
> include in virsh ?  The data is pretty useless on its own - I figure
> any app needing to access this is almost certainly not going to be
> shell scripting, and thus using one of the language bindings directly.
> Having the virsh command will probably just encourage people to use
> this as a really dumb file copy command.

This virsh changed removed.

> > +    /* The path is correct, now try to open it and get its size. */
> > +    fd = open (path, O_RDONLY);
> > +    if (fd == -1 || fstat (fd, &statbuf) == -1) {
> > +        virXendError (domain->conn, VIR_ERR_SYSTEM_ERROR, strerror (errno));
> > +        goto done;
> > +    }
> > +
> > +    /* XXX The following test fails for LVM devices for a couple
> > +     * of reasons: (1) They are commonly symlinks to /dev/mapper/foo
> > +     * and despite the man page for fstat, fstat stats the link not
> > +     * the file.  (2) Stat even on the block device returns st_size==0.
> > +     *
> > +     * Anyhow, it should be safe to ignore this test since we are
> > +     * in O_RDONLY mode.
> > +     */
> > +#if 0
> > +    /* NB we know offset > 0, size >= 0 */
> > +    if (offset + size > statbuf.st_size) {
> > +        virXendError (domain->conn, VIR_ERR_INVALID_ARG, "offset");
> > +        goto done;
> > +    }
> > +#endif
> 
> Actually the core problem is that fstat() does not return block device
> capacity. The best way to determine capacity of a block device is to
> lseek() to the end of it, and grab the return value of lseek.
> 
> There's also a Linux speciifc ioctl() to get device capacity, but 
> the lseek approach is portable.

I just removed this code.  It's not needed.

> > +
> > +    /* Seek and read. */
> > +    /* NB. Because we configure with AC_SYS_LARGEFILE, off_t should
> > +     * be 64 bits on all platforms.
> > +     */
> > +    if (lseek (fd, offset, SEEK_SET) == (off_t) -1 ||
> > +        saferead (fd, buffer, size) == (ssize_t) -1) {
> > +        virXendError (domain->conn, VIR_ERR_SYSTEM_ERROR, strerror (errno));
> > +        goto done;
> > +    }
> > +
> > +    ret = 0;
> > + done:
> > +    if (fd >= 0) close (fd);
> > +    return ret;
> > +}
> > +
> >  #endif /* ! PROXY */
> >  #endif /* WITH_XEN */
> > Index: src/xend_internal.h
> > ===================================================================
> > RCS file: /data/cvs/libvirt/src/xend_internal.h,v
> > retrieving revision 1.40
> > diff -u -r1.40 xend_internal.h
> > --- src/xend_internal.h	10 Apr 2008 16:54:54 -0000	1.40
> > +++ src/xend_internal.h	16 Apr 2008 15:18:26 -0000
> > @@ -228,6 +228,8 @@
> >  int xenDaemonDomainMigratePrepare (virConnectPtr dconn, char **cookie, int *cookielen, const char *uri_in, char **uri_out, unsigned long flags, const char *dname, unsigned long resource);
> >  int xenDaemonDomainMigratePerform (virDomainPtr domain, const char *cookie, int cookielen, const char *uri, unsigned long flags, const char *dname, unsigned long resource);
> >  
> > +int xenDaemonDomainBlockPeek (virDomainPtr domain, const char *path, long long offset, size_t size, void *buffer);
> > +
> >  #ifdef __cplusplus
> >  }
> >  #endif
> > Index: src/xm_internal.c
> > ===================================================================
> > RCS file: /data/cvs/libvirt/src/xm_internal.c,v
> > retrieving revision 1.70
> > diff -u -r1.70 xm_internal.c
> > --- src/xm_internal.c	10 Apr 2008 16:54:54 -0000	1.70
> > +++ src/xm_internal.c	16 Apr 2008 15:18:29 -0000
> > @@ -3160,4 +3160,15 @@
> >      return (ret);
> >  }
> >  
> > +int
> > +xenXMDomainBlockPeek (virDomainPtr dom,
> > +                      const char *path ATTRIBUTE_UNUSED,
> > +                      long long offset ATTRIBUTE_UNUSED,
> > +                      size_t size ATTRIBUTE_UNUSED,
> > +                      void *buffer ATTRIBUTE_UNUSED)
> > +{
> > +    xenXMError (dom->conn, VIR_ERR_NO_SUPPORT, __FUNCTION__);
> > +    return -1;
> > +}
> 
> Hmm, is there no way to share the code here with the main Xen
> driver ?  THe XenD driver impl parses the SEXPR to get the
> device info. Perhaps we could parse the XML format instead,
> then the only difference would be the API you call to get
> the XML doc in each driver. For that matter, if we worked off
> the XML format, this driver impl would be trivially sharable
> to QEMU and LXC, etc too.

I left this as it was because I'm not sure what you are suggesting.
Do you mean to do a dumpxml operation inside the driver?  Note that
we're already dumping the XML outside the driver in order to get the
paths to pass in.

Rich.

-- 
Richard Jones, Emerging Technologies, Red Hat  http://et.redhat.com/~rjones
virt-p2v converts physical machines to virtual machines.  Boot with a
live CD or over the network (PXE) and turn machines into Xen guests.
http://et.redhat.com/~rjones/virt-p2v
Index: configure.in
===================================================================
RCS file: /data/cvs/libvirt/configure.in,v
retrieving revision 1.151
diff -b -u -p -r1.151 configure.in
--- configure.in	29 May 2008 19:27:04 -0000	1.151
+++ configure.in	4 Jun 2008 15:08:08 -0000
@@ -60,6 +60,10 @@ AM_PROG_CC_C_O
 
 LIBVIRT_COMPILE_WARNINGS([maximum])
 
+dnl Support large files / 64 bit seek offsets.
+dnl Use --disable-largefile if you don't want this.
+AC_SYS_LARGEFILE
+
 dnl Availability of various common functions (non-fatal if missing).
 AC_CHECK_FUNCS([cfmakeraw regexec uname sched_getaffinity])
 
@@ -982,8 +986,6 @@ AC_SUBST([CYGWIN_EXTRA_LIBADD])
 AC_SUBST([CYGWIN_EXTRA_PYTHON_LIBADD])
 AC_SUBST([MINGW_EXTRA_LDFLAGS])
 
-AC_SYS_LARGEFILE
-
 # Set LV_LIBTOOL_OBJDIR to "." or $lt_cv_objdir, depending on whether
 # we're building shared libraries.  This is the name of the directory
 # in which .o files will be created.
Index: include/libvirt/libvirt.h
===================================================================
RCS file: /data/cvs/libvirt/include/libvirt/libvirt.h,v
retrieving revision 1.74
diff -b -u -p -r1.74 libvirt.h
--- include/libvirt/libvirt.h	23 May 2008 08:32:08 -0000	1.74
+++ include/libvirt/libvirt.h	4 Jun 2008 15:08:10 -0000
@@ -530,7 +530,11 @@ int                     virDomainInterfa
                                                  const char *path,
                                                  virDomainInterfaceStatsPtr stats,
                                                  size_t size);
-
+int                     virDomainBlockPeek (virDomainPtr dom,
+                                            const char *path,
+                                            unsigned long long offset,
+                                            size_t size,
+                                            void *buffer);
 
 /*
  * defined but not running domains
Index: include/libvirt/libvirt.h.in
===================================================================
RCS file: /data/cvs/libvirt/include/libvirt/libvirt.h.in,v
retrieving revision 1.49
diff -b -u -p -r1.49 libvirt.h.in
--- include/libvirt/libvirt.h.in	23 May 2008 08:32:08 -0000	1.49
+++ include/libvirt/libvirt.h.in	4 Jun 2008 15:08:10 -0000
@@ -530,7 +530,11 @@ int                     virDomainInterfa
                                                  const char *path,
                                                  virDomainInterfaceStatsPtr stats,
                                                  size_t size);
-
+int                     virDomainBlockPeek (virDomainPtr dom,
+                                            const char *path,
+                                            unsigned long long offset,
+                                            size_t size,
+                                            void *buffer);
 
 /*
  * defined but not running domains
Index: src/driver.h
===================================================================
RCS file: /data/cvs/libvirt/src/driver.h,v
retrieving revision 1.47
diff -b -u -p -r1.47 driver.h
--- src/driver.h	13 May 2008 06:30:58 -0000	1.47
+++ src/driver.h	4 Jun 2008 15:08:11 -0000
@@ -226,6 +226,13 @@ typedef int
                      struct _virDomainInterfaceStats *stats);
 
 typedef int
+    (*virDrvDomainBlockPeek)
+                    (virDomainPtr domain,
+                     const char *path,
+                     unsigned long long offset, size_t size,
+                     void *buffer);
+
+typedef int
     (*virDrvDomainMigratePrepare)
                     (virConnectPtr dconn,
                      char **cookie,
@@ -337,6 +344,7 @@ struct _virDriver {
     virDrvDomainMigrateFinish	domainMigrateFinish;
     virDrvDomainBlockStats      domainBlockStats;
     virDrvDomainInterfaceStats  domainInterfaceStats;
+    virDrvDomainBlockPeek	domainBlockPeek;
     virDrvNodeGetCellsFreeMemory	nodeGetCellsFreeMemory;
     virDrvNodeGetFreeMemory		getFreeMemory;
 };
Index: src/libvirt.c
===================================================================
RCS file: /data/cvs/libvirt/src/libvirt.c,v
retrieving revision 1.143
diff -b -u -p -r1.143 libvirt.c
--- src/libvirt.c	29 May 2008 19:23:17 -0000	1.143
+++ src/libvirt.c	4 Jun 2008 15:08:14 -0000
@@ -2586,7 +2586,73 @@ virDomainInterfaceStats (virDomainPtr do
     return -1;
 }
 
+/**
+ * virDomainBlockPeek:
+ * @dom: pointer to the domain object
+ * @path: path to the block device
+ * @offset: offset within block device
+ * @size: size to read
+ * @buffer: return buffer (must be at least size bytes)
+ *
+ * This function allows you to read the contents of a domain's
+ * disk device.
+ *
+ * Typical uses for this are to determine if the domain has
+ * written a Master Boot Record (indicating that the domain
+ * has completed installation), or to try to work out the state
+ * of the domain's filesystems.
+ *
+ * (Note that in the local case you might try to open the
+ * block device or file directly, but that won't work in the
+ * remote case, nor if you don't have sufficient permission.
+ * Hence the need for this call).
+ *
+ * 'path' must be a device or file corresponding to the domain.
+ * In other words it must be the precise string returned in
+ * a <disk><source dev='...'/></disk> from
+ * virDomainGetXMLDesc.
+ *
+ * 'offset' and 'size' represent an area which must lie entirely
+ * within the device or file.  'size' may be 0 to test if the
+ * call would succeed.
+ *
+ * 'buffer' is the return buffer and must be at least 'size' bytes.
+ *
+ * Returns: 0 in case of success or -1 in case of failure.
+ */
+int
+virDomainBlockPeek (virDomainPtr dom,
+                    const char *path,
+                    unsigned long long offset /* really 64 bits */,
+                    size_t size,
+                    void *buffer)
+{
+    virConnectPtr conn;
+    DEBUG("domain=%p, path=%s, offset=%lld, size=%zi, buffer=%p",
+          dom, path, offset, size, buffer);
 
+    if (!VIR_IS_CONNECTED_DOMAIN (dom)) {
+        virLibDomainError (NULL, VIR_ERR_INVALID_DOMAIN, __FUNCTION__);
+        return -1;
+    }
+    conn = dom->conn;
+
+    if (!path) {
+        virLibDomainError (dom, VIR_ERR_INVALID_ARG, "path == NULL");
+        return -1;
+    }
+    /* Allow size == 0 as an access test. */
+    if (size > 0 && !buffer) {
+        virLibDomainError (dom, VIR_ERR_INVALID_ARG, "buffer == NULL");
+        return -1;
+    }
+
+    if (conn->driver->domainBlockPeek)
+        return conn->driver->domainBlockPeek (dom, path, offset, size, buffer);
+
+    virLibDomainError (dom, VIR_ERR_NO_SUPPORT, __FUNCTION__);
+    return -1;
+}
 
 
 /************************************************************************
Index: src/libvirt_sym.version
===================================================================
RCS file: /data/cvs/libvirt/src/libvirt_sym.version,v
retrieving revision 1.40
diff -b -u -p -r1.40 libvirt_sym.version
--- src/libvirt_sym.version	13 May 2008 06:30:58 -0000	1.40
+++ src/libvirt_sym.version	4 Jun 2008 15:08:14 -0000
@@ -73,6 +73,7 @@
 	virDomainSetSchedulerParameters;
 	virDomainBlockStats;
 	virDomainInterfaceStats;
+	virDomainBlockPeek;
 	virDomainAttachDevice;
 	virDomainDetachDevice;
 
Index: src/qemu_driver.c
===================================================================
RCS file: /data/cvs/libvirt/src/qemu_driver.c,v
retrieving revision 1.82
diff -b -u -p -r1.82 qemu_driver.c
--- src/qemu_driver.c	29 May 2008 19:20:23 -0000	1.82
+++ src/qemu_driver.c	4 Jun 2008 15:08:17 -0000
@@ -3521,6 +3521,7 @@ static virDriver qemuDriver = {
     NULL, /* domainMigrateFinish */
     qemudDomainBlockStats, /* domainBlockStats */
     qemudDomainInterfaceStats, /* domainInterfaceStats */
+    NULL, /* domainBlockPeek */
 #if HAVE_NUMACTL
     qemudNodeGetCellsFreeMemory, /* nodeGetCellsFreeMemory */
     qemudNodeGetFreeMemory,  /* getFreeMemory */
Index: src/test.c
===================================================================
RCS file: /data/cvs/libvirt/src/test.c,v
retrieving revision 1.75
diff -b -u -p -r1.75 test.c
--- src/test.c	29 May 2008 19:20:23 -0000	1.75
+++ src/test.c	4 Jun 2008 15:08:18 -0000
@@ -2060,6 +2060,7 @@ static virDriver testDriver = {
     NULL, /* domainMigrateFinish */
     NULL, /* domainBlockStats */
     NULL, /* domainInterfaceStats */
+    NULL, /* domainBlockPeek */
     testNodeGetCellsFreeMemory, /* nodeGetCellsFreeMemory */
     NULL, /* getFreeMemory */
 };
Index: src/xen_unified.c
===================================================================
RCS file: /data/cvs/libvirt/src/xen_unified.c,v
retrieving revision 1.44
diff -b -u -p -r1.44 xen_unified.c
--- src/xen_unified.c	14 May 2008 19:51:24 -0000	1.44
+++ src/xen_unified.c	4 Jun 2008 15:08:19 -0000
@@ -1235,6 +1235,29 @@ xenUnifiedDomainInterfaceStats (virDomai
 }
 
 static int
+xenUnifiedDomainBlockPeek (virDomainPtr dom, const char *path,
+                           unsigned long long offset, size_t size,
+                           void *buffer)
+{
+    int r;
+    GET_PRIVATE (dom->conn);
+
+    if (priv->opened[XEN_UNIFIED_XEND_OFFSET]) {
+        r = xenDaemonDomainBlockPeek (dom, path, offset, size, buffer);
+        if (r != -2) return r;
+        /* r == -2 means declined, so fall through to XM driver ... */
+    }
+
+    if (priv->opened[XEN_UNIFIED_XM_OFFSET]) {
+        if (xenXMDomainBlockPeek (dom, path, offset, size, buffer) == 0)
+            return 0;
+    }
+
+    xenUnifiedError (dom->conn, VIR_ERR_NO_SUPPORT, __FUNCTION__);
+    return -1;
+}
+
+static int
 xenUnifiedNodeGetCellsFreeMemory (virConnectPtr conn, unsigned long long *freeMems,
                                   int startCell, int maxCells)
 {
@@ -1329,6 +1352,7 @@ static virDriver xenUnifiedDriver = {
     .domainMigrateFinish		= xenUnifiedDomainMigrateFinish,
     .domainBlockStats	= xenUnifiedDomainBlockStats,
     .domainInterfaceStats = xenUnifiedDomainInterfaceStats,
+    .domainBlockPeek	= xenUnifiedDomainBlockPeek,
     .nodeGetCellsFreeMemory = xenUnifiedNodeGetCellsFreeMemory,
     .getFreeMemory = xenUnifiedNodeGetFreeMemory,
 };
Index: src/xend_internal.c
===================================================================
RCS file: /data/cvs/libvirt/src/xend_internal.c,v
retrieving revision 1.194
diff -b -u -p -r1.194 xend_internal.c
--- src/xend_internal.c	29 May 2008 19:20:23 -0000	1.194
+++ src/xend_internal.c	4 Jun 2008 15:08:22 -0000
@@ -18,6 +18,8 @@
 #include <sys/socket.h>
 #include <sys/un.h>
 #include <sys/errno.h>
+#include <sys/stat.h>
+#include <fcntl.h>
 #include <unistd.h>
 #include <string.h>
 #include <stdlib.h>
@@ -1705,6 +1707,219 @@ error:
     return ret;
 }
 
+typedef int
+  (*sexp_blockdevs_cb)
+  (virConnectPtr conn, void *data,
+   int isBlock, int cdrom, int isNoSrcCdrom, int hvm,
+   const char *drvName, const char *drvType,
+   const char *src, const char *dst,
+   const char *mode);
+
+/**
+ * xend_parse_sexp_blockdevs:
+ * @conn: connection
+ * @root: root sexpr
+ * @xendConfigVersion: version of xend
+ * @fn: callback function
+ * @data: optional data for callback function
+ *
+ * This parses out block devices from the domain sexpr and calls
+ * fn (conn, data, ...) for each block device found.
+ *
+ * Returns 0 if successful or -1 if failed.
+ */
+static int
+xend_parse_sexp_blockdevs (virConnectPtr conn, struct sexpr *root,
+                           int xendConfigVersion,
+                           sexp_blockdevs_cb fn, void *data)
+{
+    struct sexpr *cur, *node;
+    int hvm;
+
+    hvm = sexpr_lookup(root, "domain/image/hvm") ? 1 : 0;
+
+    for (cur = root; cur->kind == SEXPR_CONS; cur = cur->u.s.cdr) {
+        node = cur->u.s.car;
+        /* Normally disks are in a (device (vbd ...)) block
+           but blktap disks ended up in a differently named
+           (device (tap ....)) block.... */
+        if (sexpr_lookup(node, "device/vbd") ||
+            sexpr_lookup(node, "device/tap")) {
+            char *offset;
+            int ret = -1;
+            int isBlock = 0;
+            int cdrom = 0;
+            int isNoSrcCdrom = 0;
+            char *drvName = NULL;
+            char *drvType = NULL;
+            const char *src = NULL;
+            const char *dst = NULL;
+            const char *mode = NULL;
+
+            /* Again dealing with (vbd...) vs (tap ...) differences */
+            if (sexpr_lookup(node, "device/vbd")) {
+                src = sexpr_node(node, "device/vbd/uname");
+                dst = sexpr_node(node, "device/vbd/dev");
+                mode = sexpr_node(node, "device/vbd/mode");
+            } else {
+                src = sexpr_node(node, "device/tap/uname");
+                dst = sexpr_node(node, "device/tap/dev");
+                mode = sexpr_node(node, "device/tap/mode");
+            }
+
+            if (dst == NULL) {
+                virXendError(conn, VIR_ERR_INTERNAL_ERROR,
+                             _("domain information incomplete, vbd has no dev"));
+                goto bad_parse;
+            }
+
+            if (src == NULL) {
+                /* There is a case without the uname to the CD-ROM device */
+                offset = strchr(dst, ':');
+                if (offset) {
+                    if (hvm && STREQ(offset, ":cdrom")) {
+                        isNoSrcCdrom = 1;
+                    }
+                    offset[0] = '\0';
+                }
+                if (!isNoSrcCdrom) {
+                    virXendError(conn, VIR_ERR_INTERNAL_ERROR,
+                                 _("domain information incomplete, vbd has no src"));
+                    goto bad_parse;
+                }
+            }
+
+            if (!isNoSrcCdrom) {
+                offset = strchr(src, ':');
+                if (!offset) {
+                    virXendError(conn, VIR_ERR_INTERNAL_ERROR,
+                                 _("cannot parse vbd filename, missing driver name"));
+                    goto bad_parse;
+                }
+
+                if (VIR_ALLOC_N(drvName, (offset-src)+1) < 0) {
+                    virXendError(conn, VIR_ERR_NO_MEMORY,
+                                 _("allocate new buffer"));
+                    goto bad_parse;
+                }
+                strncpy(drvName, src, (offset-src));
+                drvName[offset-src] = '\0';
+
+                src = offset + 1;
+
+                if (STREQ (drvName, "tap")) {
+                    offset = strchr(src, ':');
+                    if (!offset) {
+                        virXendError(conn, VIR_ERR_INTERNAL_ERROR,
+                                     _("cannot parse vbd filename, missing driver type"));
+                        goto bad_parse;
+                    }
+
+                    if (VIR_ALLOC_N(drvType, (offset-src)+1)< 0) {
+                        virXendError(conn, VIR_ERR_NO_MEMORY,
+                                     _("allocate new buffer"));
+                        goto bad_parse;
+                    }
+                    strncpy(drvType, src, (offset-src));
+                    drvType[offset-src] = '\0';
+                    src = offset + 1;
+                    /* Its possible to use blktap driver for block devs
+                       too, but kinda pointless because blkback is better,
+                       so we assume common case here. If blktap becomes
+                       omnipotent, we can revisit this, perhaps stat()'ing
+                       the src file in question */
+                    isBlock = 0;
+                } else if (STREQ(drvName, "phy")) {
+                    isBlock = 1;
+                } else if (STREQ(drvName, "file")) {
+                    isBlock = 0;
+                }
+            }
+
+            if (STREQLEN (dst, "ioemu:", 6))
+                dst += 6;
+
+            /* New style disk config from Xen >= 3.0.3 */
+            if (xendConfigVersion > 1) {
+                offset = strrchr(dst, ':');
+                if (offset) {
+                    if (STREQ (offset, ":cdrom")) {
+                        cdrom = 1;
+                    } else if (STREQ (offset, ":disk")) {
+                        /* The default anyway */
+                    } else {
+                        /* Unknown, lets pretend its a disk too */
+                    }
+                    offset[0] = '\0';
+                }
+            }
+
+            /* Call the callback function. */
+            ret = fn (conn, data, isBlock, cdrom, isNoSrcCdrom, hvm,
+                      drvName, drvType, src, dst, mode);
+
+        bad_parse:
+            VIR_FREE(drvName);
+            VIR_FREE(drvType);
+
+            if (ret == -1) return -1;
+        }
+    }
+
+    return 0;
+}
+
+static int
+xend_parse_sexp_desc_blockdev (virConnectPtr conn ATTRIBUTE_UNUSED,
+                               void *data,
+                               int isBlock, int cdrom, int isNoSrcCdrom,
+                               int hvm,
+                               const char *drvName, const char *drvType,
+                               const char *src, const char *dst,
+                               const char *mode)
+{
+    virBuffer *buf = (virBuffer *) data;
+    const char *bus = NULL;
+
+    if (!isNoSrcCdrom) {
+        virBufferVSprintf(buf, "    <disk type='%s' device='%s'>\n",
+                          isBlock ? "block" : "file",
+                          cdrom ? "cdrom" : "disk");
+        if (drvType) {
+            virBufferVSprintf(buf, "      <driver name='%s' type='%s'/>\n", drvName, drvType);
+        } else {
+            virBufferVSprintf(buf, "      <driver name='%s'/>\n", drvName);
+        }
+        if (isBlock) {
+            virBufferVSprintf(buf, "      <source dev='%s'/>\n", src);
+        } else {
+            virBufferVSprintf(buf, "      <source file='%s'/>\n", src);
+        }
+    } else {
+        /* This case is the cdrom device only */
+        virBufferAddLit(buf, "    <disk device='cdrom'>\n");
+    }
+    if (STRPREFIX(dst, "xvd") || !hvm) {
+        bus = "xen";
+    } else if (STRPREFIX(dst, "sd")) {
+        bus = "scsi";
+    } else {
+        bus = "ide";
+    }
+    virBufferVSprintf(buf, "      <target dev='%s' bus='%s'/>\n",
+                      dst, bus);
+
+    /* XXX should we force mode == r, if cdrom==1, or assume
+       xend has already done this ? */
+    if ((mode != NULL) && (STREQ (mode, "r")))
+        virBufferAddLit(buf, "      <readonly/>\n");
+    else if ((mode != NULL) && (STREQ (mode, "w!")))
+        virBufferAddLit(buf, "      <shareable/>\n");
+    virBufferAddLit(buf, "    </disk>\n");
+
+    return 0;
+}
+
 /**
  * xend_parse_sexp_desc:
  * @conn: the connection associated with the XML
@@ -1849,164 +2064,15 @@ xend_parse_sexp_desc(virConnectPtr conn,
     if ((tmp != NULL) && (tmp[0] != 0))
         virBufferVSprintf(&buf, "    <emulator>%s</emulator>\n", tmp);
 
+    /* append block devices */
+    if (xend_parse_sexp_blockdevs (conn, root, xendConfigVersion,
+                                   xend_parse_sexp_desc_blockdev, &buf) == -1)
+        goto error;
+
+    /* append network devices and framebuffer */
     for (cur = root; cur->kind == SEXPR_CONS; cur = cur->u.s.cdr) {
         node = cur->u.s.car;
-        /* Normally disks are in a (device (vbd ...)) block
-           but blktap disks ended up in a differently named
-           (device (tap ....)) block.... */
-        if (sexpr_lookup(node, "device/vbd") ||
-            sexpr_lookup(node, "device/tap")) {
-            char *offset;
-            int isBlock = 0;
-            int cdrom = 0;
-            int isNoSrcCdrom = 0;
-            char *drvName = NULL;
-            char *drvType = NULL;
-            const char *src = NULL;
-            const char *dst = NULL;
-            const char *mode = NULL;
-            const char *bus = NULL;
-
-            /* Again dealing with (vbd...) vs (tap ...) differences */
-            if (sexpr_lookup(node, "device/vbd")) {
-                src = sexpr_node(node, "device/vbd/uname");
-                dst = sexpr_node(node, "device/vbd/dev");
-                mode = sexpr_node(node, "device/vbd/mode");
-            } else {
-                src = sexpr_node(node, "device/tap/uname");
-                dst = sexpr_node(node, "device/tap/dev");
-                mode = sexpr_node(node, "device/tap/mode");
-            }
-
-            if (dst == NULL) {
-                virXendError(conn, VIR_ERR_INTERNAL_ERROR,
-                             _("domain information incomplete, vbd has no dev"));
-                goto bad_parse;
-            }
-
-            if (src == NULL) {
-                /* There is a case without the uname to the CD-ROM device */
-                offset = strchr(dst, ':');
-                if (offset) {
-                    if (hvm && STREQ( offset , ":cdrom")) {
-                        isNoSrcCdrom = 1;
-                    }
-                    offset[0] = '\0';
-                }
-                if (!isNoSrcCdrom) {
-                    virXendError(conn, VIR_ERR_INTERNAL_ERROR,
-                                 _("domain information incomplete, vbd has no src"));
-                    goto bad_parse;
-                }
-            }
-
-            if (!isNoSrcCdrom) {
-                offset = strchr(src, ':');
-                if (!offset) {
-                    virXendError(conn, VIR_ERR_INTERNAL_ERROR,
-                                 _("cannot parse vbd filename, missing driver name"));
-                    goto bad_parse;
-                }
-
-                if (VIR_ALLOC_N(drvName, (offset-src)+1) < 0) {
-                    virXendError(conn, VIR_ERR_NO_MEMORY,
-                                 _("allocate new buffer"));
-                    goto bad_parse;
-                }
-                strncpy(drvName, src, (offset-src));
-                drvName[offset-src] = '\0';
-
-                src = offset + 1;
-
-                if (STREQ(drvName, "tap")) {
-                    offset = strchr(src, ':');
-                    if (!offset) {
-                        virXendError(conn, VIR_ERR_INTERNAL_ERROR,
-                                     _("cannot parse vbd filename, missing driver type"));
-                        goto bad_parse;
-                    }
-
-                    if (VIR_ALLOC_N(drvType, (offset-src)+1)< 0) {
-                        virXendError(conn, VIR_ERR_NO_MEMORY,
-                                     _("allocate new buffer"));
-                        goto bad_parse;
-                    }
-                    strncpy(drvType, src, (offset-src));
-                    drvType[offset-src] = '\0';
-                    src = offset + 1;
-                    /* Its possible to use blktap driver for block devs
-                       too, but kinda pointless because blkback is better,
-                       so we assume common case here. If blktap becomes
-                       omnipotent, we can revisit this, perhaps stat()'ing
-                       the src file in question */
-                    isBlock = 0;
-                } else if (STREQ(drvName, "phy")) {
-                    isBlock = 1;
-                } else if (STREQ(drvName, "file")) {
-                    isBlock = 0;
-                }
-            }
-
-            if (STRPREFIX(dst, "ioemu:"))
-                dst += 6;
-
-            /* New style disk config from Xen >= 3.0.3 */
-            if (xendConfigVersion > 1) {
-                offset = strrchr(dst, ':');
-                if (offset) {
-                    if (STREQ(offset, ":cdrom")) {
-                        cdrom = 1;
-                    } else if (STREQ(offset, ":disk")) {
-                        /* The default anyway */
-                    } else {
-                        /* Unknown, lets pretend its a disk too */
-                    }
-                    offset[0] = '\0';
-                }
-            }
-
-            if (!isNoSrcCdrom) {
-                virBufferVSprintf(&buf, "    <disk type='%s' device='%s'>\n",
-                                  isBlock ? "block" : "file",
-                                  cdrom ? "cdrom" : "disk");
-                if (drvType) {
-                    virBufferVSprintf(&buf, "      <driver name='%s' type='%s'/>\n", drvName, drvType);
-                } else {
-                    virBufferVSprintf(&buf, "      <driver name='%s'/>\n", drvName);
-                }
-                if (isBlock) {
-                    virBufferVSprintf(&buf, "      <source dev='%s'/>\n", src);
-                } else {
-                    virBufferVSprintf(&buf, "      <source file='%s'/>\n", src);
-                }
-            } else {
-                /* This case is the cdrom device only */
-                virBufferAddLit(&buf, "    <disk device='cdrom'>\n");
-            }
-
-            if (STRPREFIX(dst, "xvd") || !hvm) {
-                bus = "xen";
-            } else if (STRPREFIX(dst, "sd")) {
-                bus = "scsi";
-            } else {
-                bus = "ide";
-            }
-            virBufferVSprintf(&buf, "      <target dev='%s' bus='%s'/>\n",
-                              dst, bus);
-
-
-            /* XXX should we force mode == r, if cdrom==1, or assume
-               xend has already done this ? */
-            if ((mode != NULL) && (STREQ(mode, "r")))
-                virBufferAddLit(&buf, "      <readonly/>\n");
-            else if ((mode != NULL) && (STREQ(mode, "w!")))
-                virBufferAddLit(&buf, "      <shareable/>\n");
-            virBufferAddLit(&buf, "    </disk>\n");
-
-            bad_parse:
-            VIR_FREE(drvName);
-            VIR_FREE(drvType);
-        } else if (sexpr_lookup(node, "device/vif")) {
+        if (sexpr_lookup(node, "device/vif")) {
             const char *tmp2, *model;
             tmp2 = sexpr_node(node, "device/vif/script");
             tmp = sexpr_node(node, "device/vif/bridge");
@@ -4433,5 +4499,110 @@ error:
     return (ret);
 }
 
+struct check_path_data {
+    const char *path;
+    int ok;
+};
+
+static int
+check_path (virConnectPtr conn ATTRIBUTE_UNUSED, void *vp,
+            int isBlock ATTRIBUTE_UNUSED,
+            int cdrom, int isNoSrcCdrom,
+            int hvm ATTRIBUTE_UNUSED,
+            const char *drvName ATTRIBUTE_UNUSED,
+            const char *drvType ATTRIBUTE_UNUSED,
+            const char *src, const char *dst ATTRIBUTE_UNUSED,
+            const char *mode ATTRIBUTE_UNUSED)
+{
+    struct check_path_data *data = (struct check_path_data *) vp;
+
+    if (!isNoSrcCdrom && !cdrom && src && STREQ (src, data->path))
+        data->ok = 1;
+
+    return 0;
+}
+
+/**
+ * xenDaemonDomainBlockPeek:
+ * @dom: domain object
+ * @path: path to the file or device
+ * @offset: offset
+ * @size: size
+ * @buffer: return buffer
+ *
+ * Returns 0 if successful, -1 if error, -2 if declined.
+ */
+int
+xenDaemonDomainBlockPeek (virDomainPtr domain, const char *path,
+                          unsigned long long offset, size_t size,
+                          void *buffer)
+{
+    xenUnifiedPrivatePtr priv;
+    struct sexpr *root;
+    struct check_path_data data;
+    int fd, ret = -1;
+    struct stat statbuf;
+
+    priv = (xenUnifiedPrivatePtr) domain->conn->privateData;
+
+    if (domain->id < 0 && priv->xendConfigVersion < 3)
+        return -2;              /* Decline, allow XM to handle it. */
+
+    /* Security check: The path must correspond to a block device. */
+    if (domain->id > 0)
+        root = sexpr_get (domain->conn, "/xend/domain/%d?detail=1",
+                          domain->id);
+    else if (domain->id < 0)
+        root = sexpr_get (domain->conn, "/xend/domain/%s?detail=1",
+                          domain->name);
+    else {
+        /* This call always fails for dom0. */
+        virXendError (domain->conn, VIR_ERR_NO_SUPPORT,
+                      _("domainBlockPeek is not supported for dom0"));
+        return -1;
+    }
+
+    if (!root) {
+        virXendError (domain->conn, VIR_ERR_XEN_CALL, __FUNCTION__);
+        return -1;
+    }
+
+    data.path = path;
+    data.ok = 0;
+
+    if (xend_parse_sexp_blockdevs (domain->conn, root,
+                                   priv->xendConfigVersion,
+                                   check_path, &data) == -1)
+        return -1;
+
+    if (!data.ok) {
+        virXendError (domain->conn, VIR_ERR_INVALID_ARG,
+                      _("invalid path"));
+        return -1;
+    }
+
+    /* The path is correct, now try to open it and get its size. */
+    fd = open (path, O_RDONLY);
+    if (fd == -1 || fstat (fd, &statbuf) == -1) {
+        virXendError (domain->conn, VIR_ERR_SYSTEM_ERROR, strerror (errno));
+        goto done;
+    }
+
+    /* Seek and read. */
+    /* NB. Because we configure with AC_SYS_LARGEFILE, off_t should
+     * be 64 bits on all platforms.
+     */
+    if (lseek (fd, offset, SEEK_SET) == (off_t) -1 ||
+        saferead (fd, buffer, size) == (ssize_t) -1) {
+        virXendError (domain->conn, VIR_ERR_SYSTEM_ERROR, strerror (errno));
+        goto done;
+    }
+
+    ret = 0;
+ done:
+    if (fd >= 0) close (fd);
+    return ret;
+}
+
 #endif /* ! PROXY */
 #endif /* WITH_XEN */
Index: src/xend_internal.h
===================================================================
RCS file: /data/cvs/libvirt/src/xend_internal.h,v
retrieving revision 1.43
diff -b -u -p -r1.43 xend_internal.h
--- src/xend_internal.h	9 May 2008 08:17:19 -0000	1.43
+++ src/xend_internal.h	4 Jun 2008 15:08:22 -0000
@@ -244,6 +244,8 @@ virDomainPtr xenDaemonLookupByName(virCo
 int xenDaemonDomainMigratePrepare (virConnectPtr dconn, char **cookie, int *cookielen, const char *uri_in, char **uri_out, unsigned long flags, const char *dname, unsigned long resource);
 int xenDaemonDomainMigratePerform (virDomainPtr domain, const char *cookie, int cookielen, const char *uri, unsigned long flags, const char *dname, unsigned long resource);
 
+int xenDaemonDomainBlockPeek (virDomainPtr domain, const char *path, unsigned long long offset, size_t size, void *buffer);
+
 #ifdef __cplusplus
 }
 #endif
Index: src/xm_internal.c
===================================================================
RCS file: /data/cvs/libvirt/src/xm_internal.c,v
retrieving revision 1.80
diff -b -u -p -r1.80 xm_internal.c
--- src/xm_internal.c	3 Jun 2008 08:01:45 -0000	1.80
+++ src/xm_internal.c	4 Jun 2008 15:08:24 -0000
@@ -3241,4 +3241,15 @@ xenXMDomainDetachDevice(virDomainPtr dom
     return (ret);
 }
 
+int
+xenXMDomainBlockPeek (virDomainPtr dom,
+                      const char *path ATTRIBUTE_UNUSED,
+                      unsigned long long offset ATTRIBUTE_UNUSED,
+                      size_t size ATTRIBUTE_UNUSED,
+                      void *buffer ATTRIBUTE_UNUSED)
+{
+    xenXMError (dom->conn, VIR_ERR_NO_SUPPORT, __FUNCTION__);
+    return -1;
+}
+
 #endif /* WITH_XEN */
Index: src/xm_internal.h
===================================================================
RCS file: /data/cvs/libvirt/src/xm_internal.h,v
retrieving revision 1.11
diff -b -u -p -r1.11 xm_internal.h
--- src/xm_internal.h	10 Apr 2008 16:54:54 -0000	1.11
+++ src/xm_internal.h	4 Jun 2008 15:08:24 -0000
@@ -60,6 +60,7 @@ int xenXMDomainUndefine(virDomainPtr dom
 
 virConfPtr xenXMParseXMLToConfig(virConnectPtr conn, const char *xml);
 char *xenXMDomainFormatXML(virConnectPtr conn, virConfPtr conf);
+int xenXMDomainBlockPeek (virDomainPtr dom, const char *path, unsigned long long offset, size_t size, void *buffer);
 
 #ifdef __cplusplus
 }
Index: tests/sexpr2xmldata/sexpr2xml-curmem.xml
===================================================================
RCS file: /data/cvs/libvirt/tests/sexpr2xmldata/sexpr2xml-curmem.xml,v
retrieving revision 1.6
diff -b -u -p -r1.6 sexpr2xml-curmem.xml
--- tests/sexpr2xmldata/sexpr2xml-curmem.xml	8 May 2008 14:41:56 -0000	1.6
+++ tests/sexpr2xmldata/sexpr2xml-curmem.xml	4 Jun 2008 15:08:24 -0000
@@ -15,17 +15,17 @@
   <on_reboot>restart</on_reboot>
   <on_crash>restart</on_crash>
   <devices>
+    <disk type='file' device='disk'>
+      <driver name='tap' type='aio'/>
+      <source file='/xen/rhel5.img'/>
+      <target dev='xvda' bus='xen'/>
+    </disk>
     <interface type='bridge'>
       <source bridge='xenbr0'/>
       <target dev='vif5.0'/>
       <mac address='00:16:3e:1d:06:15'/>
       <script path='vif-bridge'/>
     </interface>
-    <disk type='file' device='disk'>
-      <driver name='tap' type='aio'/>
-      <source file='/xen/rhel5.img'/>
-      <target dev='xvda' bus='xen'/>
-    </disk>
     <input type='mouse' bus='xen'/>
     <graphics type='vnc' port='-1'/>
     <console type='pty'>
Index: tests/sexpr2xmldata/sexpr2xml-no-source-cdrom.xml
===================================================================
RCS file: /data/cvs/libvirt/tests/sexpr2xmldata/sexpr2xml-no-source-cdrom.xml,v
retrieving revision 1.10
diff -b -u -p -r1.10 sexpr2xml-no-source-cdrom.xml
--- tests/sexpr2xmldata/sexpr2xml-no-source-cdrom.xml	8 May 2008 14:41:56 -0000	1.10
+++ tests/sexpr2xmldata/sexpr2xml-no-source-cdrom.xml	4 Jun 2008 15:08:25 -0000
@@ -20,11 +20,6 @@
   <clock offset='utc'/>
   <devices>
     <emulator>/usr/lib/xen/bin/qemu-dm</emulator>
-    <interface type='bridge'>
-      <source bridge='xenbr0'/>
-      <target dev='vif6.0'/>
-      <mac address='00:16:3e:0a:7b:39'/>
-    </interface>
     <disk type='block' device='disk'>
       <driver name='phy'/>
       <source dev='/dev/sda8'/>
@@ -34,6 +29,11 @@
       <target dev='hdc' bus='ide'/>
       <readonly/>
     </disk>
+    <interface type='bridge'>
+      <source bridge='xenbr0'/>
+      <target dev='vif6.0'/>
+      <mac address='00:16:3e:0a:7b:39'/>
+    </interface>
     <input type='mouse' bus='ps2'/>
     <graphics type='vnc' port='-1'/>
     <serial type='pty'>

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