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).
J
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