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

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Message-ID: <4AA0A05B.5010806@kernel.org>
Date:	Fri, 04 Sep 2009 14:06:35 +0900
From:	Tejun Heo <tj@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
CC:	Jeremy Fitzhardinge <jeremy@...p.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

H. Peter Anvin wrote:
> On 09/03/2009 08:47 PM, Tejun Heo wrote:
>> 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.
>> Heh... here's a naive and hopeful plan.  How about we beg gcc
>> developers to allow different segment register and offset in newer gcc
>> versions and then use the same one when building with the new gcc?
>> This should solve the i386 problem too.  It would be the best as we
>> get to keep the separate segment register from the userland.  Too
>> hopeful?
> 
> I think it's possible to set the register in more recent gcc.  Doing the
> sane thing and having a symbol for an offset is probably worse.

I was thinking about altering the build process so that we can use sed
to substitute %gs:40 with %fs:40 while compiling.  If it's already
possible to override the register in more recent gcc, no need to go
into that horror.

> I can talk to H.J. Lu about this tomorrow.

Great, please keep us posted.

Thanks.

-- 
tejun
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