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

From: Ingo Molnar
Date: Fri Sep 04 2009 - 01:12:37 EST

* Tejun Heo <tj@xxxxxxxxxx> 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.

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