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SubjectRe: i386 PDA patches use of %gs

* Jeremy Fitzhardinge <jeremy@goop.org> wrote:

> Ingo Molnar wrote:
> > no, that's not what it does. It measures 50000000 switches of the _same_
> > selector value, without using any of the selectors in the loop itself.
> > I.e. no mixing at all! But when the kernel and userspace uses %gs, it's
> > the cost of switching between two selector values of %gs that has to be
> > measured. Your code does not measure that at all, AFAICS.
> >
> I think you're misreading it. This is the inner loop:
>
> for(i = 0; i < COUNT; i++) {
> asm volatile("push %%gs; mov %1, %%gs; addl $1, %%gs:%0; popl %%gs"
> : "+m" (*offset): "r" (seg) : "memory");
> sync();
> }
> return "gs";
>
> On entry, %gs will contain the normal usermode TLS selector. "seg" is
> another selector allocated with set_thread_area(). The asm pushes the
> old %gs, loads the new one, uses a memory address via the new segment,
> then restores the previous %gs.

but it does not actually use the 'normal usermode TLS selector' - it
only loads it.

a meaningful test would be to allocate two selector values and load and
read+write memory through both of them.

Ingo
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