Message-ID: <4AA08ED0.4050206@zytor.com>
Date: Thu, 03 Sep 2009 20:51:44 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Tejun Heo <tj@kernel.org>
CC: Jeremy Fitzhardinge <jeremy@goop.org>, mingo@redhat.com,
       linux-kernel@vger.kernel.org, jeremy.fitzhardinge@citrix.com,
       stable@kernel.org, tglx@linutronix.de, mingo@elte.hu,
       linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base
 is cache aligned
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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 can talk to H.J. Lu about this tomorrow.

	-hpa

-- 
H. Peter Anvin, Intel Open Source Technology Center
I work for Intel.  I don't speak on their behalf.

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