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

[Michal Privoznik]

On 8/12/19 1:18 PM, Menno Lageman wrote:
> 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 at oracle.com>
>>>
>>> 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 at oracle.com>
>>> Signed-off-by: Menno Lageman <menno.lageman at oracle.com>
>>> ---
>>>    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 @@
>>>      </hyperv>
>>>      <kvm>
>>>        <hidden state='on'/>
>>> +    <hint-dedicated state='on'/>
>>>      </kvm>
>>>      <pvspinlock state='on'/>
>>>      <gic version='2'/>
>>> @@ -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" ?

What do you mean by 'dedicated vCPU'? So far I only know dedicated 
physical CPU (or CPU core) - and that's not easy to set up, you'll need 
numa pinning, cpu pinning, isolcpus, and whatnot. Also, what 
opimizations does it enable?

I'm not saying we need to put everything in the docs, but also I don't 
want the docs to be undestandable by developers who developed given feature.

Michal