[PATCH v2 4/4] qemu: support dirty ring feature

[huangy81 at chinatelecom.cn]

From: Hyman Huang(黄勇) <huangy81 at chinatelecom.cn>

QEMU has introduced a dirty ring feature, this patch add
corresponding feature named 'dirty-ring', which enable
dirty ring feature when starting vm.

to enable the feature, libvirt add "-accel dirty-ring-size=xxx"
to QEMU command line, the following XML needs to be added to
the guest's domain description:

<features>
   <kvm>
     <dirty-ring state='on' size='xxx'>
   </kvm>
</features>

if property "state=on" but property "size" not be configured, set
default ring size with 4096.

since dirty ring can only be enabled by specifying "-accel" option
and do not support the legacy style, it seems that there's no
other way to work around this, so we use "-accel" option to specify
accelerator instead of "-machine" when building qemu commandline.

details about the qemu "-accel" option:
https://lore.kernel.org/qemu-devel/3aa73987-40e8-3619-0723-9f17f73850bd@redhat.com/

Signed-off-by: Hyman Huang(黄勇) <huangy81 at chinatelecom.cn>
---
 docs/formatdomain.rst         | 16 +++++++++-------
 docs/schemas/domaincommon.rng | 10 ++++++++++
 src/qemu/qemu_command.c       |  6 ++++++
 3 files changed, 25 insertions(+), 7 deletions(-)

diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst
index c6dede0..ffb94bd 100644
--- a/docs/formatdomain.rst
+++ b/docs/formatdomain.rst
@@ -1820,6 +1820,7 @@ Hypervisors may allow certain CPU / machine features to be toggled on/off.
        <hidden state='on'/>
        <hint-dedicated state='on'/>
        <poll-control state='on'/>
+       <dirty-ring state='on' size='4096'/>
      </kvm>
      <xen>
        <e820_host state='on'/>
@@ -1902,13 +1903,14 @@ are:
 ``kvm``
    Various features to change the behavior of the KVM hypervisor.
 
-   ============== ============================================================================ ======= ============================
-   Feature        Description                                                                  Value   Since
-   ============== ============================================================================ ======= ============================
-   hidden         Hide the KVM hypervisor from standard MSR based discovery                    on, off :since:`1.2.8 (QEMU 2.1.0)`
-   hint-dedicated Allows a guest to enable optimizations when running on dedicated vCPUs       on, off :since:`5.7.0 (QEMU 2.12.0)`
-   poll-control   Decrease IO completion latency by introducing a grace period of busy waiting on, off :since:`6.10.0 (QEMU 4.2)`
-   ============== ============================================================================ ======= ============================
+   ============== ============================================================================ ====================================================== ============================
+   Feature        Description                                                                  Value                                                  Since
+   ============== ============================================================================ ====================================================== ============================
+   hidden         Hide the KVM hypervisor from standard MSR based discovery                    on, off                                                :since:`1.2.8 (QEMU 2.1.0)`
+   hint-dedicated Allows a guest to enable optimizations when running on dedicated vCPUs       on, off                                                :since:`5.7.0 (QEMU 2.12.0)`
+   poll-control   Decrease IO completion latency by introducing a grace period of busy waiting on, off                                                :since:`6.10.0 (QEMU 4.2)`
+   dirty-ring     Enable dirty ring feature                                                    on, off; size - must be power of 2, range [1024,65536] :since:`7.5.0 (QEMU 6.1)`
+   ============== ============================================================================ ====================================================== ============================
 
 ``xen``
    Various features to change the behavior of the Xen hypervisor.
diff --git a/docs/schemas/domaincommon.rng b/docs/schemas/domaincommon.rng
index 5ea14b6..f476719 100644
--- a/docs/schemas/domaincommon.rng
+++ b/docs/schemas/domaincommon.rng
@@ -7107,6 +7107,16 @@
             <ref name="featurestate"/>
           </element>
         </optional>
+        <optional>
+          <element name="dirty-ring">
+            <ref name="featurestate"/>
+            <optional>
+              <attribute name="size">
+                <data type="unsignedInt"/>
+              </attribute>
+            </optional>
+          </element>
+        </optional>
       </interleave>
     </element>
   </define>
diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
index d1e9bee..da78873 100644
--- a/src/qemu/qemu_command.c
+++ b/src/qemu/qemu_command.c
@@ -6774,6 +6774,12 @@ qemuBuildAccelCommandLineKvmOptions(virCommand *cmd,
         g_auto(virBuffer) buf = VIR_BUFFER_INITIALIZER;
         virCommandAddArg(cmd, "-accel");
         virBufferAddLit(&buf, "kvm");
+
+        if (def->features[VIR_DOMAIN_FEATURE_KVM] == VIR_TRISTATE_SWITCH_ON &&
+            def->kvm_features[VIR_DOMAIN_KVM_DIRTY_RING] == VIR_TRISTATE_SWITCH_ON) {
+            virBufferAsprintf(&buf, ",dirty-ring-size=%d", def->dirty_ring_size);
+        }
+
         virCommandAddArgBuffer(cmd, &buf);
     }
 }
-- 
1.8.3.1