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[libvirt] [PATCH v3] Add new attribute wrpolicy to <driver> element



This introduces new attribute wrpolicy with only supported
value as immediate. This will be an optional
attribute with no defaults. This helps specify whether
to skip the host page cache.

When wrpolicy is specified, meaning when wrpolicy=immediate
a writeback is explicitly initiated for the dirty pages in
the host page cache as part of the guest file write operation.

Usage:
<filesystem type='mount' accessmode='passthrough'>
  <driver type='path' wrpolicy='immediate'/>
  <source dir='/export/to/guest'/>
  <target dir='mount_tag'/>
</filesystem>

Currently this only works with type='mount' for the QEMU/KVM driver.

Signed-off-by: Deepak C Shetty <deepakcs linux vnet ibm com>
---
v2:
- added writeout as a qemu cap
- cosmetic changes in comments
- moved to using VIR_ERR_CONFIG_UNSUPPORTED
- corrected doc

v3:
- changed attr name from writeout to wrpolicy
- moved attr to be part of driver xml element

 docs/formatdomain.html.in     |   12 ++++++++++--
 docs/schemas/domaincommon.rng |    7 +++++++
 src/conf/domain_conf.c        |   27 ++++++++++++++++++++++++++-
 src/conf/domain_conf.h        |   10 ++++++++++
 src/qemu/qemu_capabilities.c  |    3 +++
 src/qemu/qemu_capabilities.h  |    5 +++--
 src/qemu/qemu_command.c       |   14 ++++++++++++++
 7 files changed, 73 insertions(+), 5 deletions(-)

diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index 18b7e22..fb2bcbc 100644
--- a/docs/formatdomain.html.in
+++ b/docs/formatdomain.html.in
@@ -1310,7 +1310,7 @@
       &lt;target dir='/'/&gt;
     &lt;/filesystem&gt;
     &lt;filesystem type='mount' accessmode='passthrough'&gt;
-      &lt;driver type='path'/&gt;
+      &lt;driver type='path' wrpolicy='immediate'/&gt;
       &lt;source dir='/export/to/guest'/&gt;
       &lt;target dir='/import/from/host'/&gt;
       &lt;readonly/&gt;
@@ -1337,7 +1337,15 @@
         sub-element <code>driver</code>, with an
         attribute <code>type='path'</code>
         or <code>type='handle'</code> <span class="since">(since
-        0.9.7)</span>.
+        0.9.7). The driver block has an optional attribute
+        <code>wrpolicy</code> with the only supported value
+        as <code>immediate</code>. It helps specify whether to skip 
+        the host page cache. When wrpolicy is specified, meaning when 
+        wrpolicy=immediate a writeback is explicitly initiated for the
+        dirty pages in the host page cache as part of the guest file 
+        write operation. When this attribute is not specified, there 
+        are no defaults, meaning explicit writeback won't be initiated
+        </span>.
         </dd>
         <dt><code>type='template'</code></dt>
         <dd>
diff --git a/docs/schemas/domaincommon.rng b/docs/schemas/domaincommon.rng
index 353faea..810b634 100644
--- a/docs/schemas/domaincommon.rng
+++ b/docs/schemas/domaincommon.rng
@@ -1142,6 +1142,13 @@
                     <value>handle</value>
                   </choice>
                 </attribute>
+                <optional>
+                  <attribute name="wrpolicy">
+                    <choice>
+                      <value>immediate</value>
+                    </choice>
+                  </attribute>
+                </optional>
                 <empty/>
               </element>
             </optional>
diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index 0190a81..98e5866 100644
--- a/src/conf/domain_conf.c
+++ b/src/conf/domain_conf.c
@@ -257,6 +257,9 @@ VIR_ENUM_IMPL(virDomainFSAccessMode, VIR_DOMAIN_FS_ACCESSMODE_LAST,
               "mapped",
               "squash")
 
+VIR_ENUM_IMPL(virDomainFSWrpolicy, VIR_DOMAIN_FS_WRPOLICY_LAST,
+              "default",
+              "immediate")
 
 VIR_ENUM_IMPL(virDomainNet, VIR_DOMAIN_NET_TYPE_LAST,
               "user",
@@ -3468,6 +3471,7 @@ virDomainFSDefParseXML(xmlNodePtr node,
     char *source = NULL;
     char *target = NULL;
     char *accessmode = NULL;
+    char *wrpolicy = NULL;
 
     if (VIR_ALLOC(def) < 0) {
         virReportOOMError();
@@ -3517,6 +3521,7 @@ virDomainFSDefParseXML(xmlNodePtr node,
                 def->readonly = 1;
             } else if ((fsdriver == NULL) && (xmlStrEqual(cur->name, BAD_CAST "driver"))) {
                 fsdriver = virXMLPropString(cur, "type");
+                wrpolicy = virXMLPropString(cur, "wrpolicy");
             }
         }
         cur = cur->next;
@@ -3530,6 +3535,16 @@ virDomainFSDefParseXML(xmlNodePtr node,
         }
     }
 
+    if (wrpolicy) {
+        if ((def->wrpolicy = virDomainFSWrpolicyTypeFromString(wrpolicy)) < 0) {
+            virDomainReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                                 _("unknown filesystem write policy '%s'"), wrpolicy);
+            goto error;
+        }
+    } else {
+        def->wrpolicy = VIR_DOMAIN_FS_WRPOLICY_DEFAULT;
+    }
+
     if (source == NULL) {
         virDomainReportError(VIR_ERR_NO_SOURCE,
                              target ? "%s" : NULL, target);
@@ -3556,6 +3571,7 @@ cleanup:
     VIR_FREE(target);
     VIR_FREE(source);
     VIR_FREE(accessmode);
+    VIR_FREE(wrpolicy);
 
     return def;
 
@@ -10163,6 +10179,7 @@ virDomainFSDefFormat(virBufferPtr buf,
     const char *type = virDomainFSTypeToString(def->type);
     const char *accessmode = virDomainFSAccessModeTypeToString(def->accessmode);
     const char *fsdriver = virDomainFSDriverTypeTypeToString(def->fsdriver);
+    const char *wrpolicy = virDomainFSWrpolicyTypeToString(def->wrpolicy);
 
     if (!type) {
         virDomainReportError(VIR_ERR_INTERNAL_ERROR,
@@ -10182,7 +10199,15 @@ virDomainFSDefFormat(virBufferPtr buf,
                       type, accessmode);
 
     if (def->fsdriver) {
-        virBufferAsprintf(buf, "      <driver type='%s'/>\n", fsdriver);
+        virBufferAsprintf(buf, "      <driver type='%s'", fsdriver);
+
+        /* Don't generate anything if wrpolicy is set to default */
+        if (def->wrpolicy) {
+            virBufferAsprintf(buf, " wrpolicy='%s'", wrpolicy);
+        }
+
+        virBufferAddLit(buf,"/>\n");
+        
     }
 
     if (def->src) {
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index 03aa5b6..7541fbc 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -479,12 +479,21 @@ enum virDomainFSAccessMode {
     VIR_DOMAIN_FS_ACCESSMODE_LAST
 };
 
+/* Filesystem Write policy */
+enum virDomainFSWrpolicy {
+    VIR_DOMAIN_FS_WRPOLICY_DEFAULT = 0,
+    VIR_DOMAIN_FS_WRPOLICY_IMMEDIATE,
+
+    VIR_DOMAIN_FS_WRPOLICY_LAST
+};
+
 typedef struct _virDomainFSDef virDomainFSDef;
 typedef virDomainFSDef *virDomainFSDefPtr;
 struct _virDomainFSDef {
     int type;
     int fsdriver;
     int accessmode;
+    int wrpolicy;
     char *src;
     char *dst;
     unsigned int readonly : 1;
@@ -1974,6 +1983,7 @@ VIR_ENUM_DECL(virDomainControllerModelUSB)
 VIR_ENUM_DECL(virDomainFS)
 VIR_ENUM_DECL(virDomainFSDriverType)
 VIR_ENUM_DECL(virDomainFSAccessMode)
+VIR_ENUM_DECL(virDomainFSWrpolicy)
 VIR_ENUM_DECL(virDomainNet)
 VIR_ENUM_DECL(virDomainNetBackend)
 VIR_ENUM_DECL(virDomainNetVirtioTxMode)
diff --git a/src/qemu/qemu_capabilities.c b/src/qemu/qemu_capabilities.c
index 43c7578..2af16f3 100644
--- a/src/qemu/qemu_capabilities.c
+++ b/src/qemu/qemu_capabilities.c
@@ -144,6 +144,7 @@ VIR_ENUM_IMPL(qemuCaps, QEMU_CAPS_LAST,
               "ich9-ahci",
               "no-acpi",
               "fsdev-readonly",
+              "fsdev-writeout",
     );
 
 struct qemu_feature_flags {
@@ -1083,6 +1084,8 @@ qemuCapsComputeCmdFlags(const char *help,
         qemuCapsSet(flags, QEMU_CAPS_FSDEV);
         if (strstr(fsdev, "readonly"))
             qemuCapsSet(flags, QEMU_CAPS_FSDEV_READONLY);
+        if (strstr(fsdev, "writeout"))
+            qemuCapsSet(flags, QEMU_CAPS_FSDEV_WRITEOUT);
     }
     if (strstr(help, "-smbios type"))
         qemuCapsSet(flags, QEMU_CAPS_SMBIOS_TYPE);
diff --git a/src/qemu/qemu_capabilities.h b/src/qemu/qemu_capabilities.h
index c759baf..5b2f932 100644
--- a/src/qemu/qemu_capabilities.h
+++ b/src/qemu/qemu_capabilities.h
@@ -113,10 +113,11 @@ enum qemuCapsFlags {
     QEMU_CAPS_NO_SHUTDOWN       = 74, /* usable -no-shutdown */
 
     QEMU_CAPS_DRIVE_CACHE_UNSAFE = 75, /* Is cache=unsafe supported? */
-    QEMU_CAPS_PCI_ROMBAR         = 76, /* -device rombar=0|1 */
+    QEMU_CAPS_PCI_ROMBAR        = 76, /* -device rombar=0|1 */
     QEMU_CAPS_ICH9_AHCI         = 77, /* -device ich9-ahci */
     QEMU_CAPS_NO_ACPI		= 78, /* -no-acpi */
-    QEMU_CAPS_FSDEV_READONLY    =79, /* -fsdev readonly supported */
+    QEMU_CAPS_FSDEV_READONLY    = 79, /* -fsdev readonly supported */
+    QEMU_CAPS_FSDEV_WRITEOUT    = 80, /* -fsdev writeout supported */
 
     QEMU_CAPS_LAST,                   /* this must always be the last item */
 };
diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index 73f673f..ba0353e 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -108,6 +108,10 @@ VIR_ENUM_IMPL(qemuDomainFSDriver, VIR_DOMAIN_FS_DRIVER_TYPE_LAST,
               "local",
               "handle");
 
+VIR_ENUM_DECL(qemuDomainFSWrpolicy)
+VIR_ENUM_IMPL(qemuDomainFSWrpolicy, VIR_DOMAIN_FS_WRPOLICY_LAST,
+              "default",
+              "immediate");
 
 static void
 uname_normalize (struct utsname *ut)
@@ -2086,6 +2090,7 @@ char *qemuBuildFSStr(virDomainFSDefPtr fs,
 {
     virBuffer opt = VIR_BUFFER_INITIALIZER;
     const char *driver = qemuDomainFSDriverTypeToString(fs->fsdriver);
+    const char *wrpolicy = qemuDomainFSWrpolicyTypeToString(fs->wrpolicy);
 
     if (fs->type != VIR_DOMAIN_FS_TYPE_MOUNT) {
         qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
@@ -2119,6 +2124,15 @@ char *qemuBuildFSStr(virDomainFSDefPtr fs,
         }
     }
 
+    if (fs->wrpolicy != VIR_DOMAIN_FS_WRPOLICY_DEFAULT) {
+       if (qemuCapsGet(qemuCaps, QEMU_CAPS_FSDEV_WRITEOUT)) {
+           virBufferAsprintf(&opt, ",writeout=%s", wrpolicy);
+       } else {
+           qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
+                          _("filesystem writeout not supported"));
+       }
+    }
+
     virBufferAsprintf(&opt, ",id=%s%s", QEMU_FSDEV_HOST_PREFIX, fs->info.alias);
     virBufferAsprintf(&opt, ",path=%s", fs->src);
 


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