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

[zhenwei pi]

On 7/2/21 4:35 AM, 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...
Hi, this patch wants to fix the issue:
AMD CPU (Rome/Milan) should get the cpuid level 0x10, not 0x1F in the 
guest. If QEMU reports a 0x1F to guest, guest(Linux) would use leaf 0x1F 
instead of leaf 0xB to get extended topology:

https://github.com/torvalds/linux/blob/master/arch/x86/kernel/cpu/topology.c#L49

static int detect_extended_topology_leaf(struct cpuinfo_x86 *c)
{
        if (c->cpuid_level >= 0x1f) {
                if (check_extended_topology_leaf(0x1f) == 0)
                        return 0x1f;
        }

        if (c->cpuid_level >= 0xb) {
                if (check_extended_topology_leaf(0xb) == 0)
                        return 0xb;
        }

        return -1;
}

Because of the wrong cpuid level, the guest gets unexpected topology 
from leaf 0x1F.

I tested https://github.com/mdroth/qemu/commits/new-cpu-types-upstream, 
and it seems that these patches could not fix this issue.

> > > 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
> > > >
> > > >   

--
zhenwei pi