[libvirt] [RFC] docs: Discourage usage of cache mode=passthrough

[Daniel P. Berrange]

On Thu, Sep 21, 2017 at 01:14:04PM -0400, Laine Stump wrote:
> On 09/19/2017 03:37 PM, Eduardo Habkost wrote:
> > Cache mode=passthrough can result in a broken cache topology if
> > the domain topology is not exactly the same as the host topology.
> > Warn about that in the documentation.
> >
> > Bug report for reference:
> > https://bugzilla.redhat.com/show_bug.cgi?id=1184125
> >
> > Signed-off-by: Eduardo Habkost <ehabkost at redhat.com>
> > ---
> >  docs/formatdomain.html.in | 4 +++-
> >  1 file changed, 3 insertions(+), 1 deletion(-)
> >
> > diff --git a/docs/formatdomain.html.in b/docs/formatdomain.html.in
> > index 57ec2ff34..9c21892f3 100644
> > --- a/docs/formatdomain.html.in
> > +++ b/docs/formatdomain.html.in
> > @@ -1478,7 +1478,9 @@
> >  
> >                <dt><code>passthrough</code></dt>
> >                <dd>The real CPU cache data reported by the host CPU will be
> > -                passed through to the virtual CPU.</dd>
> > +                passed through to the virtual CPU.  Using this mode is not
> > +                recommended unless the domain CPU and NUMA topology is exactly
> > +                the same as the host CPU and NUMA topology.</dd>
> 
> To me this sounds like it should be forbidden by libvirt, rather than
> just documented as "bad". (I haven't followed any previous discussion on
> the topic though, so maybe I'm over-reacting).

In high performance setups, people pin guest vCPUs to host pCPUs and
set the vCPU topology to match the host pCPU topology they've pinned
to. So ohaving a cache mode that matches this topology is just fine.
It simply isn't something you want as a default for the more typical
floating vCPUs scenarios.

Regards,
Daniel
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