Re: [Xen-devel] [PATCH] x86emul: correct far branch handling for 64-bit mode

[Jan Beulich <jbeulich@xxxxxxxx>]

On 13.12.2019 14:01, Andrew Cooper wrote:
> On 13/12/2019 12:54, Jan Beulich wrote:
>> AMD and friends explicitly specify that 64-bit operands aren't possible
>> for these insns. Nevertheless REX.W isn't fully ignored: It still
>> cancels a possible operand size override (0x66). Intel otoh explicitly
>> provides for 64-bit operands on the respective insn page of the SDM.
>>
>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

Thanks.

>> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
>> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
>> @@ -2519,9 +2519,16 @@ x86_decode_onebyte(
>>          case 6: /* push */
>>              if ( mode_64bit() && op_bytes == 4 )
>>                  op_bytes = 8;
>> -            /* fall through */
>> +            state->desc = DstNone | SrcMem | Mov;
>> +            break;
>> +
>>          case 3: /* call (far, absolute indirect) */
>>          case 5: /* jmp (far, absolute indirect) */
>> +            /* REX.W ignored on a vendor-dependent basis. */
>> +            if ( op_bytes == 8 &&
>> +                 (ctxt->cpuid->x86_vendor &
>> +                  (X86_VENDOR_AMD | X86_VENDOR_HYGON)) )
> 
> I'm wondering whether in general we want some amd_like() and
> intel_like() predicates.  It is how almost all of the boundaries end up
> falling.

I've been wondering the same, but didn't do so yet because while
for Hygon we can be pretty sure it's AMD-like, I'm not sure how
far we could go with covering other than Intel as Intel-like.
But yes, perhaps we could start with just amd_like() (I don't
particularly like this name, but I also can't think of anything
I would like better).

Jan

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