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

[Michael Roth]

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...

> 
> > 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
> > > 
> > >  
> > 
> 
>