Re: [PATCH] i386/kvm: add NoNonArchitecturalCoreSharing Hyper-V enlightenment

[Eduardo Habkost <ehabkost@redhat.com>]

On Mon, Oct 21, 2019 at 06:26:14PM +0200, Paolo Bonzini wrote:
> On 21/10/19 16:09, Vitaly Kuznetsov wrote:
> >>> +    if (cpu->hyperv_no_nonarch_cs == ON_OFF_AUTO_ON) {
> >>> +        env->features[FEAT_HV_RECOMM_EAX] |= HV_NO_NONARCH_CORESHARING;
> >>> +    } else if (cpu->hyperv_no_nonarch_cs == ON_OFF_AUTO_AUTO) {
> >> Do you want to make auto the default if "-cpu host,migratable=off"?  It
> >> can be done on top so I started queueing this patch.
> > Hm, one thing is that CPUID 0x40000004 doesn't exist if no Hyper-V
> > enlightenments are passed so we'll probably have to modify your idea to
> > "-cpu host,migratable=off,+any-hyperv-enlightenment" but then the
> > question is how conservative are we, like if QEMU command line doesn't
> > change can new CPUID flags appear or not? And we'll probably need a way
> > to explicitly disable HV_NO_NONARCH_CORESHARING if needed.
> 
> I would defer to Eduardo on whether "migratable=off" would allow adding
> new CPUID flags.  The follow-up question however is whether we would
> benefit from a "+hyperv" option that enables all known Hyper-V
> enlightenment for a given machine type.

I'm not sure what "adding new CPUID flags" means exactly, but on
both cases, the answer is yes:

If you mean having new flags appear with the same QEMU command
line, this is 100% OK with "-cpu host".  Doubly so with
"migratable=off".  "-cpu host" doesn't guarantee a stable guest
ABI, and migratable=off doesn't guarantee the ability to live
migrate.

If you just mean the ability to write "-cpu
host,migratable=off,+some-extra-flag", that's OK too.

I would try to make "-cpu host,migratable=off" enable all
features out of the box (because users probably expect that).
But we you have a compelling reason to not enable the hyperv
flags by default (do we?), it's OK to require something like
"-cpu host,...,+hyperv".

-- 
Eduardo