Re: [tip:x86/asm] x86/i386: Make sure stack-protector segment baseis cache aligned
From: Tejun Heo
Date: Fri Sep 04 2009 - 01:07:19 EST
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
> 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.
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