Date: Fri, 4 Sep 2009 07:12:02 +0200
From: Ingo Molnar <mingo@elte.hu>
To: Tejun Heo <tj@kernel.org>
Cc: "H. Peter Anvin" <hpa@zytor.com>, Jeremy Fitzhardinge <jeremy@goop.org>,
       mingo@redhat.com, linux-kernel@vger.kernel.org,
       jeremy.fitzhardinge@citrix.com, stable@kernel.org, tglx@linutronix.de,
       linux-tip-commits@vger.kernel.org
Subject: Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment base
	is cache aligned
Message-ID: <20090904051202.GA25714@elte.hu>
References: <tip-1ea0d14e480c245683927eecc03a70faf06e80c8@git.kernel.org> <4AA02687.9080406@zytor.com> <4AA02B02.7080101@goop.org> <4AA031DE.2070109@zytor.com> <4AA080A0.7010804@kernel.org> <4AA08283.5020306@kernel.org> <4AA08B09.50503@zytor.com> <4AA08DD3.5010509@kernel.org> <4AA08ED0.4050206@zytor.com> <4AA0A05B.5010806@kernel.org>
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* Tejun Heo <tj@kernel.org> wrote:

> 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.

Yeah - if then this should definitely be handled in the compiler.

	Ingo
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