Re: [PATCH v2] x86emul: adjust BSF/BSR/LZCNT/TZCNT behavior as to EFLAGS

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 14/07/2025 5:02 pm, Jan Beulich wrote:
> SDM revision 087 points out that apparently as of quite some time ago on
> Intel hardware BSF and BSR may alter all arithmetic flags, not just ZF.
> Because of the inconsistency (and because documentation doesn't look to

It's probably worth saying errata explicitly.  There are a whole bunch
of Intel CPUs where the behaviour doesn't match CPUID.

> be quite right about PF), best we can do is simply take the flag values
> from what the processor produces, just like we do for various other
> arithmetic insns. (Note also that AMD and Intel have always been
> disagreeing on arithmetic flags other than ZF.) To be both safe (against
> further anomalies) and consistent, extend this to {L,T}ZCNT as well.
> (Emulating the two insns correctly even when underlying hardware doesn't
> support it was perhaps nice, but yielded guest-observable
> inconsistencies.)
>
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>

Reviewed-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

This is one of the more messy parts of x86, and that's saying something.

> ---
> v2: Use emulate_2op_SrcV_srcmem() also for {L,T}ZCNT.
>
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -5270,62 +5270,26 @@ x86_emulate(
>          break;
>  
>      case X86EMUL_OPC(0x0f, 0xbc): /* bsf or tzcnt */
> -    {
> -        bool zf;
> -
> -        asm ( "bsf %2,%0" ASM_FLAG_OUT(, "; setz %1")
> -              : "=r" (dst.val), ASM_FLAG_OUT("=@ccz", "=qm") (zf)
> -              : "rm" (src.val) );
> -        _regs.eflags &= ~X86_EFLAGS_ZF;
> -        if ( (vex.pfx == vex_f3) && vcpu_has_bmi1() )
> -        {
> -            _regs.eflags &= ~X86_EFLAGS_CF;
> -            if ( zf )
> -            {
> -                _regs.eflags |= X86_EFLAGS_CF;
> -                dst.val = op_bytes * 8;
> -            }
> -            else if ( !dst.val )
> -                _regs.eflags |= X86_EFLAGS_ZF;
> -        }
> -        else if ( zf )
> +        if ( vex.pfx == vex_f3 )
> +            emulate_2op_SrcV_srcmem("rep; bsf", src, dst, _regs.eflags);

Do we need the ; ?

We surely don't on 4.21, but I presume there are bugs in older
binutils?  (All Clangs back to 3.5 seem happy)

~Andrew