linux-kernel - Re: [RFC 00/15] x86_64: Optimize percpu accesses

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Date:	Wed, 09 Jul 2008 13:05:33 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
CC:	Mike Travis <travis@....com>, Ingo Molnar <mingo@...e.hu>,
	Andrew Morton <akpm@...ux-foundation.org>,
	"H. Peter Anvin" <hpa@...or.com>,
	Christoph Lameter <cl@...ux-foundation.org>,
	Jack Steiner <steiner@....com>, linux-kernel@...r.kernel.org
Subject: Re: [RFC 00/15] x86_64: Optimize percpu accesses

Eric W. Biederman wrote:
> I just took a quick look at how stack_protector works on x86_64.  Unless there is
> some deep kernel magic that changes the segment register to %gs from the ABI specified
> %fs CC_STACKPROTECTOR is totally broken on x86_64.  We access our pda through %gs.
>   

-mcmodel=kernel switches it to using %gs.

> Further -fstack-protector-all only seems to detect against buffer overflows and
> thus corruption of the stack.  Not stack overflows.  So it doesn't appear especially
> useful.
>   

It's a bit useful.  But at the cost of preventing a pile of more useful 
unification work, not to mention making all access to per-cpu variables 
more expensive.

> So we don't we kill the broken CONFIG_CC_STACKPROTECTOR.  Stop trying to figure out
> how to use a zero based percpu area.
>   

Yes, please.

> That should allow us to make the current pda a per cpu variable, and use %gs with
> a large offset to access the per cpu area.  And since it is only the per cpu accesses
> and the pda accesses that will change we should not need to fight toolchain issues
> and other weirdness.  The linked binary can remain the same.
>   

Yes, and it would be functionally identical to the 32-bit code.

    J
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