Re: i386 PDA patches use of %gs

[Jeremy Fitzhardinge <jeremy@goop.org>]

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