Re: [PATCH] x86emul: adjust BSF/BSR behavior as to EFLAGS

[Jan Beulich <jbeulich@xxxxxxxx>]

On 23.04.2025 13:09, Jan Beulich wrote:
> On 23.04.2025 11:49, Andrew Cooper wrote:
>> On 23/04/2025 7:13 am, Jan Beulich wrote:
>>> @@ -5286,25 +5284,23 @@ x86_emulate(
>>>              else if ( !dst.val )
>>>                  _regs.eflags |= X86_EFLAGS_ZF;
>>>          }
>>> -        else if ( zf )
>>> +        else
>>>          {
>>> -            _regs.eflags |= X86_EFLAGS_ZF;
>>> -            dst.type = OP_NONE;
>>> +            emulate_2op_SrcV_srcmem("bsf", src, dst, _regs.eflags);
>>> +            if ( _regs.eflags & X86_EFLAGS_ZF )
>>> +                dst.type = OP_NONE;
>>
>> On Intel, BSF/BSR writes back the destination register.  Notably, it
>> gets 0 extended per normal rules,
> 
> That's also only on "older processors", as per the other footnote.
> 
>> which is why you have to be extra
>> careful when using the trick of preloading it with -1; the result must
>> be interpreted as (int) even over a 64bit operation.
>>
>> This needs an amd_like() qualification to override dst.type.  This
>> aspect genuinely is different between them.  Alternatively, we might be
>> able to set the operand size always to 64 and write back the entire
>> register as the processor gave to us, but I'm not sure if that will have
>> effects elsewhere.
> 
> Besides (as per above), amd_like() not covering all cases, this would
> then further need special treatment for 16-bit opsize. Plus promoting
> to 64-bit would require manually clipping the result to 5 bits when
> the original size wants 64-bit. That's imo far more complications
> than gains.

I guess I viewed this as more complicated than it is: We'd merely need
to zero-extend the source operand from opsize to 64 bits. The one
uncertainty there would be whether the undefined arithmetic flags
couldn't be dependent on operand size.

Tangential to this: Software having probed for the specific behavior of
the two insns (EFLAGS and/or destination register setting for a zero
source) will break when migrated. Therefore in principle we ought to
prevent such migrations. Thoughts there?

Jan