[libvirt PATCH 1/2] qemu: support kvm-poll-control performance hint
Tim Wiederhake
twiederh at redhat.com
Tue Nov 17 13:26:18 UTC 2020
On Mon, 2020-11-16 at 14:27 +0100, Michal Privoznik wrote:
> On 11/13/20 9:49 AM, Tim Wiederhake wrote:
> > QEMU version 4.2 introduced a performance feature under commit
> > d645e13287 ("kvm: i386: halt poll control MSR support").
> >
> > This patch adds a new KVM feature 'poll-control' to set this
> > performance
> > hint for KVM guests. The feature is off by default.
> >
> > To enable this hint and have libvirt add "-cpu host,kvm-poll-
> > control=on"
> > to the QEMU command line, the following XML code needs to be added
> > to the
> > guest's domain description:
> >
> > <features>
> > <kvm>
> > <poll-control state='on'/>
> > </kvm>
> > </features>
> >
> > Signed-off-by: Tim Wiederhake <twiederh at redhat.com>
> > ---
> > docs/formatdomain.rst | 14 ++++++++------
> > docs/schemas/domaincommon.rng | 5 +++++
> > src/conf/domain_conf.c | 4 ++++
> > src/conf/domain_conf.h | 1 +
> > src/qemu/qemu_command.c | 5 +++++
> > 5 files changed, 23 insertions(+), 6 deletions(-)
> >
> > diff --git a/docs/formatdomain.rst b/docs/formatdomain.rst
> > index a6ab845f92..cd5d7aae56 100644
> > --- a/docs/formatdomain.rst
> > +++ b/docs/formatdomain.rst
> > @@ -1766,6 +1766,7 @@ Hypervisors may allow certain CPU / machine
> > features to be toggled on/off.
> > <kvm>
> > <hidden state='on'/>
> > <hint-dedicated state='on'/>
> > + <poll-control='on'/>
> > </kvm>
> > <xen>
> > <e820_host state='on'/>
> > @@ -1848,12 +1849,13 @@ 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)`
> > - ==============
> > ===================================================================
> > === ======= ============================
> > + ==============
> > ===================================================================
> > ========= ======= ============================
> > + 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:`6.11.0 (QEMU 4.2)`
> > + ==============
> > ===================================================================
> > ========= ======= ============================
> >
> > ``xen``
> > Various features to change the behavior of the Xen hypervisor.
> > diff --git a/docs/schemas/domaincommon.rng
> > b/docs/schemas/domaincommon.rng
> > index e0d09f9c03..f86a854863 100644
> > --- a/docs/schemas/domaincommon.rng
> > +++ b/docs/schemas/domaincommon.rng
> > @@ -6512,6 +6512,11 @@
> > <ref name="featurestate"/>
> > </element>
> > </optional>
> > + <optional>
> > + <element name="poll-control">
> > + <ref name="featurestate"/>
> > + </element>
> > + </optional>
> > </interleave>
> > </element>
> > </define>
> > diff --git a/src/conf/domain_conf.c b/src/conf/domain_conf.c
> > index 9199771dc0..2b56993845 100644
> > --- a/src/conf/domain_conf.c
> > +++ b/src/conf/domain_conf.c
> > @@ -208,6 +208,7 @@ VIR_ENUM_IMPL(virDomainKVM,
> > VIR_DOMAIN_KVM_LAST,
> > "hidden",
> > "hint-dedicated",
> > + "poll-control",
> > );
> >
> > VIR_ENUM_IMPL(virDomainXen,
> > @@ -19823,6 +19824,7 @@ virDomainFeaturesDefParse(virDomainDefPtr
> > def,
> > switch ((virDomainKVM) feature) {
> > case VIR_DOMAIN_KVM_HIDDEN:
> > case VIR_DOMAIN_KVM_DEDICATED:
> > + case VIR_DOMAIN_KVM_POLLCONTROL:
> > if (!(tmp = virXMLPropString(nodes[i],
> > "state"))) {
> > virReportError(VIR_ERR_XML_ERROR,
> > _("missing 'state'
> > attribute for "
> > @@ -23982,6 +23984,7 @@
> > virDomainDefFeaturesCheckABIStability(virDomainDefPtr src,
> > switch ((virDomainKVM) i) {
> > case VIR_DOMAIN_KVM_HIDDEN:
> > case VIR_DOMAIN_KVM_DEDICATED:
> > + case VIR_DOMAIN_KVM_POLLCONTROL:
> > if (src->kvm_features[i] != dst->kvm_features[i])
> > {
> > virReportError(VIR_ERR_CONFIG_UNSUPPORTED,
> > _("State of KVM feature '%s'
> > differs: "
> > @@ -29695,6 +29698,7 @@ virDomainDefFormatFeatures(virBufferPtr
> > buf,
> > switch ((virDomainKVM) j) {
> > case VIR_DOMAIN_KVM_HIDDEN:
> > case VIR_DOMAIN_KVM_DEDICATED:
> > + case VIR_DOMAIN_KVM_POLLCONTROL:
> > if (def->kvm_features[j])
> > virBufferAsprintf(&childBuf, "<%s
> > state='%s'/>\n",
> > virDomainKVMTypeToStrin
> > g(j),
> > diff --git a/src/conf/domain_conf.h b/src/conf/domain_conf.h
> > index 16c050a3ea..3e3d4bd002 100644
> > --- a/src/conf/domain_conf.h
> > +++ b/src/conf/domain_conf.h
> > @@ -1912,6 +1912,7 @@ typedef enum {
> > typedef enum {
> > VIR_DOMAIN_KVM_HIDDEN = 0,
> > VIR_DOMAIN_KVM_DEDICATED,
> > + VIR_DOMAIN_KVM_POLLCONTROL,
> >
> > VIR_DOMAIN_KVM_LAST
> > } virDomainKVM;
>
> Until here, the change is okay.
>
> > diff --git a/src/qemu/qemu_command.c b/src/qemu/qemu_command.c
> > index 0eec35da16..34b5746c1a 100644
> > --- a/src/qemu/qemu_command.c
> > +++ b/src/qemu/qemu_command.c
> > @@ -6464,6 +6464,11 @@ qemuBuildCpuCommandLine(virCommandPtr cmd,
> > virBufferAddLit(&buf, ",kvm-hint-
> > dedicated=on");
> > break;
> >
> > + case VIR_DOMAIN_KVM_POLLCONTROL:
> > + if (def->kvm_features[i] ==
> > VIR_TRISTATE_SWITCH_ON)
> > + virBufferAddLit(&buf, ",kvm-poll-control=on");
> > + break;
> > +
> > /* coverity[dead_error_begin] */
> > case VIR_DOMAIN_KVM_LAST:
> > break;
> >
>
> But this should go into the next patch IMO. The reason is that if
> somebody wants to backport only XML part they should not be forced
> to
> backport QEMU too. In this specific case it probably doesn't doesn't
> matter that much, because no other driver uses KVM features. But I
> think
> it's still wort it (e.g. to gain the habit).
>
> In theory, src/conf/ (plus docs/rng change) can be viewed as
> "frontend"
> and driver implementation can be viewed as "backend".
> What is also acceptable is having src/conf + xml2xml tests in one
> patch,
> because xml2xml tests do test the change made to src/conf. So in
> this
> specific case, it is also acceptable to have src/conf/ and docs/rng
> change and tests/.../*.xml changes in one patch, qemu + *.args in the
> other.
>
> I'm sorry for being picky. The change itself is okay.
>
> Michal
Thanks for the review! I don't quite understand though, if you could
please clarify what you mean:
If the patch changing src/conf/domain_conf.{c,h} goes in first, we have
one commit where "<poll-control state='on'/>" would be accepted but
silently discarded, potentially messing with people git-bisect'ing in
the future.
If the patch changing src/qemu/qemu_command.c goes in first, we need
VIR_DOMAIN_KVM_POLLCONTROL defined in src/conf/domain_conf.h, thus
breaking the split between "frontend" and "backend" changes as
described above.
The patches in this series were modeled after
commits cb12c59dac04c34dc8250d93e63516b1a1af5260
and 63b2e57cb3b34568f36ad6f0c8c4244e292a679e by the way, which made the
split in the same fashion as I did.
Thanks in advance for you advice!
Tim
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