Re: [PATCH] i386: pass CLZERO to guests with EPYC CPU model on AMD ZEN platform

[Eduardo Habkost <ehabkost@redhat.com>]

On Wed, Dec 18, 2019 at 12:53:45PM +0100, Paolo Bonzini wrote:
> On 18/12/19 10:05, Ani Sinha wrote:
> > CLZERO CPUID should be passed on to the guests that use EPYC or EPYC-IBPB CPU
> > model when the AMD ZEN based host supports it. This change makes it recognize
> > this CPUID for guests which use EPYC or EPYC-IBPB CPU model.

Can you clarify what's the intended use case here?  Why the
"if host supports it" conditional?

If you need host-dependent CPU configuration, "-cpu host" (or the
libvirt "host-model" mode) is the most appropriate solution.

> > 
> > Signed-off-by: Ani Sinha <ani.sinha@nutanix.com>
> > ---
> >  target/i386/cpu.c | 2 ++
> >  1 file changed, 2 insertions(+)
> > 
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > index 69f518a..55f0691 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -3813,6 +3813,8 @@ static X86CPUDefinition builtin_x86_defs[] = {
> >              CPUID_EXT3_MISALIGNSSE | CPUID_EXT3_SSE4A | CPUID_EXT3_ABM |
> >              CPUID_EXT3_CR8LEG | CPUID_EXT3_SVM | CPUID_EXT3_LAHF_LM |
> >              CPUID_EXT3_TOPOEXT,
> > +        .features[FEAT_8000_0008_EBX] =
> > +            CPUID_8000_0008_EBX_CLZERO,
> >          .features[FEAT_7_0_EBX] =
> >              CPUID_7_0_EBX_FSGSBASE | CPUID_7_0_EBX_BMI1 | CPUID_7_0_EBX_AVX2 |
> >              CPUID_7_0_EBX_SMEP | CPUID_7_0_EBX_BMI2 | CPUID_7_0_EBX_RDSEED |
> > 
> 
> This needs to be done only for newer machine type (or is it CPU model
> versions now? need Eduardo to respond).

If we want to add it, it has to be done as a new CPU model version.

But I don't know yet if we want to add it.  Do all EPYC CPUs have
CLZERO available?  If not, it's probably not advisable to add it
to EPYC (even if it's just on EPYC-v3).

-- 
Eduardo