Re: [PATCH 2/2] i386/cpu: Mask off unsupported XSAVE components

[Xiaoyao Li]

On 7/16/2020 11:14 PM, Eduardo Habkost wrote:
> On Thu, Jul 16, 2020 at 04:20:19PM +0800, Xiaoyao Li wrote:
> > When setting up XSAVE components, it needs to mask off those unsupported
> > by KVM.
> >
> > Signed-off-by: Xiaoyao Li <xiaoyao.li@intel.com>
>
> We must never disable CPUID features silently based on host
> capabilities, otherwise we can't guarantee guest ABI stability
> when migrating to another host.  Filtering of features should
> involve a call to mark_unavailable_features() (or some equivalent
> mechanism) so we can report the missing features properly through
> QMP.
>
> Could you explain what's the bug you are trying to fix?  The loop
> at x86_cpu_filter_features() is already supposed to disable
> features unsupported by the host.

Sorry, I forgot x86_cpu_filter_features() totally when code inspection.

> > ---
> >   target/i386/cpu.c | 6 ++++--
> >   1 file changed, 4 insertions(+), 2 deletions(-)
> >
> > diff --git a/target/i386/cpu.c b/target/i386/cpu.c
> > index f5f11603e805..efc92334b7b1 100644
> > --- a/target/i386/cpu.c
> > +++ b/target/i386/cpu.c
> > @@ -6274,8 +6274,10 @@ static void x86_cpu_enable_xsave_components(X86CPU *cpu)
> >           }
> >       }
> >   
> > -    env->features[FEAT_XSAVE_COMP_LO] = mask;
> > -    env->features[FEAT_XSAVE_COMP_HI] = mask >> 32;
> > +    env->features[FEAT_XSAVE_COMP_LO] = mask &
> > +        x86_cpu_get_supported_feature_word(FEAT_XSAVE_COMP_LO, 
> > cpu->migratable);
> > +    env->features[FEAT_XSAVE_COMP_HI] = (mask >> 32) &
> > +        x86_cpu_get_supported_feature_word(FEAT_XSAVE_COMP_HI, 
> > cpu->migratable);
> >   }
> >   
> >   /***** Steps involved on loading and filtering CPUID data
> > --
> > 2.18.4