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

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Date:	Fri, 04 Sep 2009 09:01:35 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Tejun Heo <tj@...nel.org>, mingo@...hat.com,
	linux-kernel@...r.kernel.org, jeremy.fitzhardinge@...rix.com,
	stable@...nel.org, tglx@...utronix.de, mingo@...e.hu,
	linux-tip-commits@...r.kernel.org
Subject: Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base
 is cache aligned

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
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