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[libvirt] [PATCH v2] Add new attribute writeout to <filesystem> element



This introduces new attribute writeout 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 writeout is specified, meaning when writeout=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' writeout='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

 docs/formatdomain.html.in     |    9 ++++++++-
 docs/schemas/domaincommon.rng |    5 +++++
 src/conf/domain_conf.c        |   29 +++++++++++++++++++++++++++--
 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, 70 insertions(+), 5 deletions(-)

diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
index 9cf0f12..93f754a 100644
--- a/docs/formatdomain.html.in
+++ b/docs/formatdomain.html.in
@@ -1303,7 +1303,7 @@
       &lt;source name='my-vm-template'/&gt;
       &lt;target dir='/'/&gt;
     &lt;/filesystem&gt;
-    &lt;filesystem type='mount' accessmode='passthrough'&gt;
+    &lt;filesystem type='mount' accessmode='passthrough' writeout='immediate'&gt;
       &lt;driver type='path'/&gt;
       &lt;source dir='/export/to/guest'/&gt;
       &lt;target dir='/import/from/host'/&gt;
@@ -1379,6 +1379,13 @@
         </dd>
         </dl>
 
+      The filesystem block has an optional attribute <code>writeout</code> with the only 
+      supported value as <code>immediate</code>. It helps specify whether to skip the host page cache.
+      When writeout is specified, meaning when writeout=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.
+
       </dd>
 
       <dt><code>source</code></dt>
diff --git a/docs/schemas/domaincommon.rng b/docs/schemas/domaincommon.rng
index 553a6f0..0b37f05 100644
--- a/docs/schemas/domaincommon.rng
+++ b/docs/schemas/domaincommon.rng
@@ -1106,6 +1106,11 @@
           <value>squash</value>
         </choice>
         </attribute>
+        <attribute name="writeout">
+        <choice>
+          <value>immediate</value>
+        </choice>
+        </attribute>
       </optional>
     </element>
   </define>
diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
index 495ed33..a548b90 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(virDomainFSWriteout, VIR_DOMAIN_FS_WRITEOUT_LAST,
+              "default",
+              "immediate")
 
 VIR_ENUM_IMPL(virDomainNet, VIR_DOMAIN_NET_TYPE_LAST,
               "user",
@@ -3297,6 +3300,7 @@ virDomainFSDefParseXML(xmlNodePtr node,
     char *source = NULL;
     char *target = NULL;
     char *accessmode = NULL;
+    char *writeout = NULL;
 
     if (VIR_ALLOC(def) < 0) {
         virReportOOMError();
@@ -3325,6 +3329,17 @@ virDomainFSDefParseXML(xmlNodePtr node,
         def->accessmode = VIR_DOMAIN_FS_ACCESSMODE_PASSTHROUGH;
     }
 
+    writeout = virXMLPropString(node, "writeout");
+    if (writeout) {
+        if ((def->writeout = virDomainFSWriteoutTypeFromString(writeout)) < 0) {
+            virDomainReportError(VIR_ERR_CONFIG_UNSUPPORTED,
+                                 _("unknown filesystem writeout '%s'"), writeout);
+            goto error;
+        }
+    } else {
+        def->writeout = VIR_DOMAIN_FS_WRITEOUT_DEFAULT;
+    }
+
     cur = node->children;
     while (cur != NULL) {
         if (cur->type == XML_ELEMENT_NODE) {
@@ -3387,6 +3402,7 @@ cleanup:
     VIR_FREE(target);
     VIR_FREE(source);
     VIR_FREE(accessmode);
+    VIR_FREE(writeout);
 
     return def;
 
@@ -10046,6 +10062,7 @@ virDomainFSDefFormat(virBufferPtr buf,
     const char *type = virDomainFSTypeToString(def->type);
     const char *accessmode = virDomainFSAccessModeTypeToString(def->accessmode);
     const char *fsdriver = virDomainFSDriverTypeTypeToString(def->fsdriver);
+    const char *writeout = virDomainFSWriteoutTypeToString(def->writeout);
 
     if (!type) {
         virDomainReportError(VIR_ERR_INTERNAL_ERROR,
@@ -10062,12 +10079,20 @@ virDomainFSDefFormat(virBufferPtr buf,
     if (def->fsdriver == VIR_DOMAIN_FS_DRIVER_TYPE_PATH ||
         def->fsdriver == VIR_DOMAIN_FS_DRIVER_TYPE_DEFAULT) {
         virBufferAsprintf(buf,
-                          "    <filesystem type='%s' accessmode='%s'>\n",
+                          "    <filesystem type='%s' accessmode='%s'",
                           type, accessmode);
     } else {
         virBufferAsprintf(buf,
-                          "    <filesystem type='%s'>\n", type);
+                          "    <filesystem type='%s'", type);
     }
+ 
+    /* Don't generate anything if writeout is set to default */
+    if (def->writeout) {
+        virBufferAsprintf(buf, " writeout='%s'", writeout);
+    }
+
+    /* close the filesystem element */
+    virBufferAddLit(buf,">\n");
 
     if (def->fsdriver) {
         virBufferAsprintf(buf, "      <driver type='%s'/>\n", fsdriver);
diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
index 1f6e442..6f65b4a 100644
--- a/src/conf/domain_conf.h
+++ b/src/conf/domain_conf.h
@@ -459,12 +459,21 @@ enum virDomainFSAccessMode {
     VIR_DOMAIN_FS_ACCESSMODE_LAST
 };
 
+/* Filesystem Writeout */
+enum virDomainFSWriteout {
+    VIR_DOMAIN_FS_WRITEOUT_DEFAULT = 0,
+    VIR_DOMAIN_FS_WRITEOUT_IMMEDIATE,
+
+    VIR_DOMAIN_FS_WRITEOUT_LAST
+};
+
 typedef struct _virDomainFSDef virDomainFSDef;
 typedef virDomainFSDef *virDomainFSDefPtr;
 struct _virDomainFSDef {
     int type;
     int fsdriver;
     int accessmode;
+    int writeout;
     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(virDomainFSWriteout)
 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 76f3632..adfc738 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(qemuDomainFSWriteout)
+VIR_ENUM_IMPL(qemuDomainFSWriteout, VIR_DOMAIN_FS_WRITEOUT_LAST,
+              "default",
+              "immediate");
 
 static void
 uname_normalize (struct utsname *ut)
@@ -2084,6 +2088,7 @@ char *qemuBuildFSStr(virDomainFSDefPtr fs,
 {
     virBuffer opt = VIR_BUFFER_INITIALIZER;
     const char *driver = qemuDomainFSDriverTypeToString(fs->fsdriver);
+    const char *writeout = qemuDomainFSWriteoutTypeToString(fs->writeout);
 
     if (fs->type != VIR_DOMAIN_FS_TYPE_MOUNT) {
         qemuReportError(VIR_ERR_CONFIG_UNSUPPORTED, "%s",
@@ -2108,6 +2113,15 @@ char *qemuBuildFSStr(virDomainFSDefPtr fs,
             virBufferAddLit(&opt, ",security_model=none");
         }
     }
+
+    if (fs->writeout != VIR_DOMAIN_FS_WRITEOUT_DEFAULT) {
+       if (qemuCapsGet(qemuCaps, QEMU_CAPS_FSDEV_WRITEOUT)) {
+           virBufferAsprintf(&opt, ",writeout=%s", writeout);
+       } 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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