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

From: Jeremy Fitzhardinge
Date: Fri Sep 04 2009 - 12:01:41 EST

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.

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