Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment baseis cache aligned

From: Jeremy Fitzhardinge
Date: Thu Sep 03 2009 - 17:28:37 EST

On 09/03/09 14:15, H. Peter Anvin wrote:
> 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.

I'm assuming that stack-protector has fairly serious performance impact
anyway, so a bit of extra entry/exit cost is acceptable. But I agree
that there's no point in making it gratuitously bad.

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