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

On 12-08-19 13:39, Michal Privoznik wrote:
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 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'.

        <hint-dedicated state='on'/>
    <cpu mode='host-passthrough ... />

Signed-off-by: Wim ten Have <wim ten have oracle com>
Signed-off-by: Menno Lageman <menno lageman 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 @@
       &lt;hidden state='on'/&gt;
+    &lt;hint-dedicated state='on'/&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>
+          <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 tried to extract the essence of the QEMU commit message ("Add KVM_HINTS_DEDICATED performance hint, guest checks this feature bit to determine if they run on dedicated vCPUs, allowing optimizations such as usage of qspinlocks.") without adding too much detail about exactly what optimizations are available since it is the guest that does (or does not) enable these optimizations.


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.


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