Re: [libvirt] [PATCH 1/2] qemu: support for kvm-hint-dedicated performance hint

On 12-08-19 11:23, Michal Privoznik wrote: > On 8/9/19 5:19 PM, Menno Lageman wrote: >> From: Wim ten Have <wim.ten.have(a)oracle.com&gt; >> >> QEMU version 2.12.1 introduced a performance feature under commit >> be7773268d98 ("target-i386: add KVM_HINTS_DEDICATED performance hint") >> >> This patch adds a new KVM feature 'hint-dedicated' 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-hint-dedicated=on" >> to the QEMU command line, the following XML code needs to be added to >> the >> guest's domain description in conjunction with CPU >> mode='host-passthrough'. >> >>     <features> >>       <kvm> >>         <hint-dedicated state='on'/> >>       </kvm> >>     </features> >>     ... >>     <cpu mode='host-passthrough ... /> >> >> Signed-off-by: Wim ten Have <wim.ten.have(a)oracle.com&gt; >> Signed-off-by: Menno Lageman <menno.lageman(a)oracle.com&gt; >> --- >>    docs/formatdomain.html.in     |  7 +++++++ >>    docs/schemas/domaincommon.rng |  5 +++++ >>    src/conf/domain_conf.c        |  4 ++++ >>    src/conf/domain_conf.h        |  1 + >>    src/qemu/qemu_command.c       | 13 +++++++++++++ >>    5 files changed, 30 insertions(+) >> >> diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in >> index 6d084d7c0472..c9b18b57e1fc 100644 >> --- a/docs/formatdomain.html.in >> +++ b/docs/formatdomain.html.in >> @@ -2044,6 +2044,7 @@ >>      &lt;/hyperv&gt; >>      &lt;kvm&gt; >>        &lt;hidden state='on'/&gt; >> +    &lt;hint-dedicated state='on'/&gt; >>      &lt;/kvm&gt; >>      &lt;pvspinlock state='on'/&gt; >>      &lt;gic version='2'/&gt; >> @@ -2217,6 +2218,12 @@ >>              <td>on, off</td> >>              <td><span class="since">1.2.8 (QEMU 2.1.0)</span></td> >>            </tr> >> +        <tr> >> +          <td>hint-dedicated</td> >> +          <td>Set the "dedicated physical CPU" performance hint >> ("-cpu kvm-hint-dedicated=on")</td> > > I'd rather not document the command line libvirt generates here. Not > only it's an internal thing of libvirt, it'd also be a promise we > might not keep up (if qemu changes way how this is configured for > instance). > But what we should document is what this feature actually is. I've > tried to dig out KVM patches and now I have a faint idea, but we can't > expect our users to go through patches when deciding whether to turn > this on or not. How about replacing it with "Allows a guest to enable optimizations when running on dedicated vCPU" ?