Re: [PATCH] target/i386: Fix cpuid level for AMD

[Michael Roth]

On Fri, Jul 02, 2021 at 07:50:03AM +0100, David Edmondson wrote:
> On Thursday, 2021-07-01 at 15:35:49 -05, Michael Roth wrote:
> 
> > Quoting Igor Mammedov (2021-07-01 03:43:13)
> >> On Wed, 30 Jun 2021 14:18:09 -0500
> >> Michael Roth <michael.roth@amd.com> wrote:
> >> 
> >> > Quoting Dr. David Alan Gilbert (2021-06-29 09:06:02)
> >> > > * zhenwei pi (pizhenwei@bytedance.com) wrote:  
> >> > > > A AMD server typically has cpuid level 0x10(test on Rome/Milan), it
> >> > > > should not be changed to 0x1f in multi-dies case.
> >> > > > 
> >> > > > Fixes: a94e1428991 (target/i386: Add CPUID.1F generation support
> >> > > > for multi-dies PCMachine)
> >> > > > Signed-off-by: zhenwei pi <pizhenwei@bytedance.com>  
> >> > > 
> >> > > (Copying in Babu)
> >> > > 
> >> > > Hmm I think you're right.  I've cc'd in Babu and Wei.
> >> > > 
> >> > > Eduardo: What do we need to do about compatibility, do we need to wire
> >> > > this to machine type or CPU version?  
> >> > 
> >> > FWIW, there are some other CPUID entries like leaves 2 and 4 that are
> >> > also Intel-specific. With SEV-SNP CPUID enforcement, advertising them to
> >> > guests will result in failures when host SNP firmware checks the
> >> > hypervisor-provided CPUID values against the host-supported ones.
> >> > 
> >> > To address this we've been planning to add an 'amd-cpuid-only' property
> >> > to suppress them:
> >> > 
> >> >   
> >> > https://github.com/mdroth/qemu/commit/28d0553fe748d30a8af09e5e58a7da3eff03e21b
> >> > 
> >> > My thinking is this property should be off by default, and only defined
> >> > either via explicit command-line option, or via new CPU types. We're also
> >> > planning to add new CPU versions for EPYC* CPU types that set this
> >> > 'amd-cpuid-only' property by default:
> >> > 
> >> >   https://github.com/mdroth/qemu/commits/new-cpu-types-upstream
> >> It look like having new cpu versions is enough to change behavior,
> >> maybe keep 'amd-cpuid-only' as internal field and not expose it to users
> >> as a property.
> >
> > Hmm, I defined it as a property mainly to make use of
> > X86CPUVersionDefinition.props to create new versions of the CPU types
> > with those properties set.
> >
> > There's a patch there that adds X86CPUVersionDefinition.cache_info so
> > that new cache definitions can be provided for new CPU versions. So
> > would you suggest a similar approach here, e.g. adding an
> > X86CPUVersionDefinition.amd_cpuid_only field that could be used directly
> > rather than going through X86CPUVersionDefinition.props?
> >
> > There's also another new "amd-xsave" prop in that series that does something
> > similar to "amd-cpuid-only", so a little worried about tacking to much extra
> > into X86CPUVersionDefinition. But maybe that one could just be rolled into
> > "amd-cpuid-only" since it is basically fixing up xsave-related cpuid
> > entries for AMD...
> 
> The XSAVE changes are similar to
> https://lore.kernel.org/r/20210520145647.3483809-1-david.edmondson@oracle.com,
> in response to which Paolo suggested that I should have QEMU observe the
> CPUID 0xd leaves rather than encoding knowledge about the state offsets
> (at least, that's how I understood his comment).
> 
> I have patches that do this (which includes making X86XSaveArea a
> TCG-only thing) that I plan to send out in the next couple of days. They
> should make "amd-xsave" unnecessary.

Ok, that does sounds like the better approach. Thanks for the heads up.

> 
> >> 
> >> > So in general I think maybe this change should be similarly controlled by
> >> > this proposed 'amd-cpuid-only' property. Maybe for this particular case 
> >> > it's
> >> > okay to do it unconditionally, but it sounds bad to switch up the valid 
> >> > CPUID
> >> > range after a guest has already booted (which might happen with old->new
> >> > migration for instance), since it might continue treating values in the 
> >> > range
> >> > as valid afterward (but again, not sure that's the case here or not).
> >> > 
> >> > There's some other changes with the new CPU types that we're still
> >> > considering/testing internally, but should be able to post them in some 
> >> > form
> >> > next week.
> >> > 
> >> > -Mike
> >> > 
> >> > > 
> >> > > Dave
> >> > >   
> >> > > > ---
> >> > > >  target/i386/cpu.c | 8 ++++++--
> >> > > >  1 file changed, 6 insertions(+), 2 deletions(-)
> >> > > > 
> >> > > > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> >> > > > index a9fe1662d3..3934c559e4 100644
> >> > > > --- a/target/i386/cpu.c
> >> > > > +++ b/target/i386/cpu.c
> >> > > > @@ -5961,8 +5961,12 @@ void x86_cpu_expand_features(X86CPU *cpu, 
> >> > > > Error **errp)
> >> > > >              }
> >> > > >          }
> >> > > >  
> >> > > > -        /* CPU topology with multi-dies support requires 
> >> > > > CPUID[0x1F] */
> >> > > > -        if (env->nr_dies > 1) {
> >> > > > +        /*
> >> > > > +         * Intel CPU topology with multi-dies support requires 
> >> > > > CPUID[0x1F].
> >> > > > +         * For AMD Rome/Milan, cpuid level is 0x10, and guest OS 
> >> > > > should detect
> >> > > > +         * extended toplogy by leaf 0xB. Only adjust it for Intel 
> >> > > > CPU.
> >> > > > +         */
> >> > > > +        if ((env->nr_dies > 1) && IS_INTEL_CPU(env)) {
> >> > > >              x86_cpu_adjust_level(cpu, &env->cpuid_min_level, 0x1F);
> >> > > >          }
> >> > > >  
> >> > > > -- 
> >> > > > 2.25.1
> >> > > > 
> >> > > >   
> >> > > -- 
> >> > > Dr. David Alan Gilbert / dgilbert@redhat.com / Manchester, UK
> >> > > 
> >> > >  
> >> > 
> >> 
> >>
> 
> dme.
> -- 
> I had my eyes closed in the dark.
>