Re: [Qemu-devel] KVM guest cpu L3 cache and cpufreq

[Eduardo Habkost]

On Tue, Aug 27, 2013 at 03:18:16PM +0200, Benoît Canet wrote:
[...]
> > > Do you have some recommandations regarding the other fields of the edx 
> > > register ?
> > 
> > Probably it will be a good idea to let each CPU model have their own
> > defaults for the cache information.
> 
> Do you mean querying the host CPUID registers and forwarding the values to the
> guest ?

No, I mean that it would make sense to have "-cpu 486" expose a
different cache topology from "-cpu Haswell" by default.

But a "forward host cache information to guest" mode may be useful, too,
and it could be the default mode for "-cpu host".

> 
> > 
> > > What would be an acceptable user interface to set this ?
> > 
> > Things I can remember from the top of my head:
> > 
> >  * It would be interesting to redo this patch, before doing anything else:
> >    
> > https://github.com/ehabkost/qemu-hacks/commit/10b675fa1269a65a553586545fdbcd95ace79d65
> >  * We need to keep compatibility on existing machine-types (not change
> >    the cache size);
> >  * If some information is reported on multiple CPUID leaves, we need to
> >    keep them consistent with each other (see the FIXME comments on the
> >    URL above);
> I could write a patch to fix broken CPUID leaves.
> 
> But how would it work regarding compatibility ? Is it right to just fix the
> leaves or does the machine type rule apply here ?

Yes, if you change anything guest-visible, you need to keep
compatibility on older machine-types.

-- 
Eduardo