[PATCH v6 4/4] qemu: support dirty ring feature

[Peter Krempa]

On Sat, Nov 20, 2021 at 03:20:48 -0500, huangy81 at chinatelecom.cn wrote:
> From: Hyman Huang(黄勇) <huangy81 at chinatelecom.cn>
> 
> dirty ring feature was introduced in qemu-6.1, 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         | 18 ++++++++++--------
>  docs/schemas/domaincommon.rng | 10 ++++++++++
>  src/qemu/qemu_command.c       |  6 ++++++
>  3 files changed, 26 insertions(+), 8 deletions(-)
> 
> diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst
> index eb8c973cf1..ea69b61c70 100644
> --- a/docs/formatdomain.rst
> +++ b/docs/formatdomain.rst
> @@ -1843,6 +1843,7 @@ Hypervisors may allow certain CPU / machine features to be toggled on/off.
>         <hint-dedicated state='on'/>
>         <poll-control state='on'/>
>         <pv-ipi state='off'/>
> +       <dirty-ring state='on' size='4096'/>
>       </kvm>
>       <xen>
>         <e820_host state='on'/>
> @@ -1925,14 +1926,15 @@ 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)`
> -   pv-ipi         Paravirtualized send IPIs                                                    on, off :since:`7.10.0 (QEMU 3.1)`
> -   ============== ============================================================================ ======= ============================
> +   ============== ============================================================================ ====================================================== ============================
> +   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)`
> +   pv-ipi         Paravirtualized send IPIs                                                    on, off                                                :since:`7.10.0 (QEMU 3.1)`
> +   dirty-ring     Enable dirty ring feature                                                    on, off; size - must be power of 2, range [1024,65536] :since:`7.10.0 (QEMU 6.1)`
> +   ============== ============================================================================ ====================================================== ============================
>  
>  ``xen``
>     Various features to change the behavior of the Xen hypervisor.

Okay, so both hunks belong actually to the previous commit.

> diff --git a/docs/schemas/domaincommon.rng b/docs/schemas/domaincommon.rng
> index f01b7a6470..5f9fe3cc58 100644
> --- a/docs/schemas/domaincommon.rng
> +++ b/docs/schemas/domaincommon.rng
> @@ -7212,6 +7212,16 @@
>              <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>

And this one too.

> diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
> index 145596d11a..863876bfae 100644
> --- a/src/qemu/qemu_command.c
> +++ b/src/qemu/qemu_command.c
> @@ -7043,6 +7043,12 @@ 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);
>      }
>  }

A test case is needed both for qemuxml2argvtest and qemuxml2xmltest.