Re: [PATCH v3] x86/emul: Adjust handling of CR8_LEGACY

[Teddy Astie <teddy.astie@xxxxxxxxxx>]

Le 08/07/2026 à 11:37, Jan Beulich a écrit :
> On 08.07.2026 11:29, Teddy Astie wrote:
> > Le 07/07/2026 à 21:49, Andrew Cooper a écrit :
> > > --- a/xen/arch/x86/x86_emulate/decode.c
> > > +++ b/xen/arch/x86/x86_emulate/decode.c
> > > @@ -780,12 +780,12 @@ decode_twobyte(struct x86_emulate_state *s,
> > >            break;
> > >    
> > >        case 0x20: case 0x22: /* mov to/from cr */
> > > -        if ( s->lock_prefix && vcpu_has_cr8_legacy() )
> > > +        if ( s->lock_prefix && vcpu_has_cr8_legacy() && s->modrm_reg == 0 )
> > >            {
> > > -            s->modrm_reg += 8;
> > > +            s->modrm_reg = 8;
> > >                s->lock_prefix = false;
> > >            }
> > > -        /* fall through */
> > > +        fallthrough;
> > >        case 0x21: case 0x23: /* mov to/from dr */
> > >            ASSERT(s->ea.type == OP_REG); /* Early operand adjustment ensures 
> > > this. */
> > >            generate_exception_if(s->lock_prefix, X86_EXC_UD);
> > I'm fine with it; though we probably want to make sure we always
> > advertise the AltMovCr8 feature bit as APM suggest users to always check
> > for this before relying on this behavior.
> Why would we set the flag when hardware doesn't have it? Note the
> vcpu_has_cr8_legacy() check there - we only engage this behavior if
> the guest sees the feature as available.
Hmm, ok, though this bit is named as "AltMovCr8" in July 2025 APM; but 
we name it "cr8_legacy". Has the name of the feature bit changed in APM 
over time ?
> Jan
Teddy

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