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Message-ID: <4AA139DF.3050902@goop.org>
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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