Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base is cache aligned

Jeremy Fitzhardinge
2009-09-04T16:01:34Z

On 09/03/09 20:35, H. Peter Anvin wrote:
> On 09/03/2009 07:59 PM, Tejun Heo wrote:
>
>> Another question. Other than saving and loading an extra segment
>> register on kernel entry/exit, whether using the same or different
>> segment registers doesn't look like would make difference
>> performance-wise. If I'm interpreting the wording in the optimization
>> manual correctly, it means that each non-zero segment based memory
>> access will be costly regardless of which specific segment register is
>> in use and there's no way we can merge segment based dereferences for
>> stackprotector and percpu variables.
>>
>>
> It's correct that it doesn't make any difference for access, only for load.
>

Well, to be completely precise, restore. When returning to usermode,
the "pop %seg" is slightly faster if you're restoring a null selector,
which is typically the case for %fs as 32-bit usermode doesn't use it.

J