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

[Eduardo Habkost]

On Thu, Sep 28, 2017 at 09:21:41AM +0100, Daniel P. Berrange wrote:
> 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.

So, should this patch be applied?

-- 
Eduardo