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

[Daniel P. Berrange]

On Mon, Nov 06, 2017 at 11:43:49AM -0200, Eduardo Habkost wrote:
> On Mon, Nov 06, 2017 at 01:17:02PM +0000, Daniel P. Berrange wrote:
> > On Mon, Nov 06, 2017 at 11:10:00AM -0200, Eduardo Habkost wrote:
> > > 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?
> > 
> > We could take a patch that describes more clearly when it is reasonable
> > to use the passthrough mode.
> 
> Why "unless the domain CPU and NUMA topology is exactly the same
> as the host CPU and NUMA topology" isn't a clear description?

Just matching topology is not useful unless you've also pinned the
guest CPUs to host CPUs. So I think it'd be clearer to say something
like

  "If using 'passthrough' mode, it is recommended to explicitly pin each
   virtual CPU to a dedicated host CPU, and setup the guest CPU and NUMA
   topology to match that of the host. Mis-matched topology or freely
   floating CPUs will result in unpredictable performance, so should be
   avoided."

Regards,
Daniel
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