Re: [PATCH v2 5/8] x86/pv: allow reading FEATURE_CONTROL MSR

["Roger Pau Monné" <roger.pau@citrix.com>]

On Thu, Aug 27, 2020 at 05:53:16PM +0200, Jan Beulich wrote:
> On 20.08.2020 17:08, Roger Pau Monne wrote:
> > @@ -181,6 +182,18 @@ int guest_rdmsr(struct vcpu *v, uint32_t msr, uint64_t *val)
> >          /* Not offered to guests. */
> >          goto gp_fault;
> >  
> > +    case MSR_IA32_FEATURE_CONTROL:
> > +        if ( !(cp->x86_vendor & X86_VENDOR_INTEL) )
> > +            goto gp_fault;
> 
> Can we really do it this way round, rather than raising #GP when
> we know the MSR isn't there (AMD / Hygon)? I realized code e.g.
> ...
> 
> > +        *val = IA32_FEATURE_CONTROL_LOCK;
> > +        if ( vmce_has_lmce(v) )
> > +            *val |= IA32_FEATURE_CONTROL_LMCE_ON;
> > +        if ( nestedhvm_enabled(d) )
> > +            *val |= IA32_FEATURE_CONTROL_ENABLE_VMXON_OUTSIDE_SMX;
> > +        break;
> > +
> > +
> >      case MSR_IA32_PLATFORM_ID:
> >          if ( !(cp->x86_vendor & X86_VENDOR_INTEL) ||
> >               !(boot_cpu_data.x86_vendor & X86_VENDOR_INTEL) )
> 
> ... in context right here does it the same way, but I still
> wonder whether we wouldn't better switch existing instances, too.

Hm, no idea really. Right now it seems better to check for != Intel
rather than AMD | Hygon | Centaur | Shanghai, as that's a MSR specific
to Intel.

Do those MSRs exist in Centaur / Shanghai?

Thanks, Roger.