Message-ID: <4AA031DE.2070109@zytor.com>
Date: Thu, 03 Sep 2009 14:15:10 -0700
From: "H. Peter Anvin" <hpa@zytor.com>
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To: Jeremy Fitzhardinge <jeremy@goop.org>
CC: 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,
       Tejun Heo <tj@kernel.org>
Subject: Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base
 is cache aligned
References: <4AA01893.6000507@goop.org> <tip-1ea0d14e480c245683927eecc03a70faf06e80c8@git.kernel.org> <4AA02687.9080406@zytor.com> <4AA02B02.7080101@goop.org>
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On 09/03/2009 01:45 PM, Jeremy Fitzhardinge wrote:
> 
> Two problems:
> 
>     * gcc generates %gs: references for stack-protector, but we use %fs
>       for percpu data (because restoring %fs is faster if it's a null
>       selector; TLS uses %gs).  I guess we could use %fs if
>       !CONFIG_CC_STACKPROTECTOR, or %gs if we are using it (though that
>       has some fiddly ramifications for things like ptrace).

Well, by touching two segments we're getting the worst of both worlds,
so at least assuming some significant number of real-world deployments
use CC_STACKPROTECTOR, we really don't want to pessimize that case too much.

>     * The i386 percpu %fs base is offset by -__per_cpu_start from the
>       percpu variables, so we can directly refer to %fs:per_cpu__foo. 
>       I'm not sure what it would take to unify i386 to use the same
>       scheme as x86-64.

OK, I was under the impression that that had already been done (and no,
I didn't bother to look at the code.)  I guess I was wrong (and yes,
this is an absolute precondition.)

> Neither looks insoluble.

Agreed.  Looks like something that can and probably should be done but
is a bit further out.

	-hpa
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