Messages in this thread | | | Date | Wed, 15 Nov 2006 19:26:14 +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: > >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.
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