Re: [PATCH RFC 0/6] Implement per-processor data areas for i386.

[Jeremy Fitzhardinge]

Andi Kleen wrote:
> On AMD K7/K8 a segment register prefix is a single cycle penalty.
>
> I couldn't find anything in the Intel optimization manuals on it, but I assume
> it's also not dramatic.
>   

All I could find was:

   * avoid multiple prefixes (which was the least important guideline
     in instruction selection)
   * avoid using multiple segment registers (the pentium M only has one
     level of segment register renaming)
   * avoid prefixes which take the instruction length over 7 bytes

None of these apply to the use of %gs to access PDA.

Most of the discussion about prefixes is in avoiding the 0x66 16-bit prefix.

   J
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@xxxxxxxxxxxxxxx
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/