Re: [Xen-devel] [PATCH] x86/emul: only emulate possibly operand sizes for POPA

[Keir Fraser <keir@xxxxxxx>]

On 07/11/2012 16:08, "Jan Beulich" <JBeulich@xxxxxxxx> wrote:

> This opcode neither support 1-byte operands, nor does it support 8-byte
> ones (since the opcode is undefined in 64-bit mode). Simplify the code
> accordingly.
> 
> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
> 
> --- a/xen/arch/x86/x86_emulate/x86_emulate.c
> +++ b/xen/arch/x86/x86_emulate/x86_emulate.c
> @@ -1996,13 +1996,10 @@ x86_emulate(
>              if ( (rc = read_ulong(x86_seg_ss, sp_post_inc(op_bytes),
>                                    &dst.val, op_bytes, ctxt, ops)) != 0 )
>                  goto done;
> -            switch ( op_bytes )
> -            {
> -            case 1: *(uint8_t  *)regs[i] = (uint8_t)dst.val; break;
> -            case 2: *(uint16_t *)regs[i] = (uint16_t)dst.val; break;
> -            case 4: *regs[i] = (uint32_t)dst.val; break; /* 64b: zero-ext */
> -            case 8: *regs[i] = dst.val; break;
> -            }
> +            if ( op_bytes != 2 )
> +                *regs[i] = (uint32_t)dst.val; /* 64b: zero-ext */
> +            else
> +                *(uint16_t *)regs[i] = (uint16_t)dst.val;

Would prefer:
 if ( op_bytes == 2 )
     *(uint16_t *)regs[i] = (uint16_t)dst.val;
 else
     *regs[i] = dst.val;

Handles the exceptional case immediately after its predicate. And the cast
from uint32_t, and 64b-related comment, are pointless and in fact misleading
in the default case, since as you say the instruction is invalid in 64-bit
mode.

Apart from that:
Acked-by: Keir Fraser <keir@xxxxxxx>

 -- Keir

>          }
>          break;
>      }
> 
> 
> 
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