* Ingo Molnar <[email protected]> wrote:
> > My test program does the same thing, except using GDT entries 6 and
> > 7 (selectors 0x33 and 0x3b).
>
> 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.
for example, your test_fs() code does:
for(i = 0; i < COUNT; i++) {
asm volatile("push %%fs; mov %1, %%fs; addl $1, %%fs:%0; popl %%fs"
: "+m" (*offset): "r" (seg) : "memory");
sync();
}
that loads (and uses) a single selector value for %fs, and doesnt do any
mixed use as far as i can see.
Ingo
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