* Jeremy Fitzhardinge <[email protected]> wrote:
> Ingo Molnar wrote:
> >well, the most important thing i believe you didnt test: the effect of
> >mixing two descriptors on the _same_ selector: one %gs selector value
> >loaded and used by glibc, and another %gs selector value loaded and used
> >by the kernel, intermixed. It's the mixing that causes the descriptor
> >cache reload. (unless i missed some detail about your testcase)
>
> But it doesn't mix different descriptors on the same selector; the GDT
> is initialized when the CPU is brought up, and is unchanged from then
> on. The PDA descriptor is GDT entry 27 and the userspace TLS entries
> are 6-8, so in the typical case %gs will alternate between 0x33 and
> 0xd8 as it enters and leaves the kernel.
>
> 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.
Ingo
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