Messages in this thread | | | Date | Wed, 15 Nov 2006 19:43:15 +0100 | From | Ingo Molnar <> | Subject | Re: i386 PDA patches use of %gs |
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* 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.
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