Messages in this thread | | | Date | Wed, 04 Jun 2008 06:48:32 -0700 | From | Mike Travis <> | Subject | Re: [PATCH 3/4] x86_64: Fold pda into per cpu area |
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Jeremy Fitzhardinge wrote: > Mike Travis wrote: >> * Declare the pda as a per cpu variable. >> >> * Make the x86_64 per cpu area start at zero. >> >> * Since the pda is now the first element of the per_cpu area, cpu_pda() >> is no longer needed and per_cpu() can be used instead. This also >> makes >> the _cpu_pda[] table obsolete. >> >> * Since %gs is pointing to the pda, it will then also point to the >> per cpu >> variables and can be accessed thusly: >> >> %gs:[&per_cpu_xxxx - __per_cpu_start] >>
The above is only a partial story (I folded the two patches but didn't update the comments correctly.] The variables are already offset from __per_cpu_start by virtue of the .data.percpu section being based at zero. Therefore only the %gs register needs to be set to the base of each cpu's percpu section to resolve the target address:
%gs:&per_cpu_xxxx
And the .data.percpu.first forces the pda percpu variable to the front.
> > Unfortunately that doesn't actually work, because you can't have a reloc > with two variables. > > In something like: > > mov %gs:per_cpu__foo - 12345, %rax > mov %gs:per_cpu__foo, %rax > mov %gs:per_cpu__foo - 12345(%rip), %rax > mov %gs:per_cpu__foo(%rip), %rax > mov %gs:per_cpu__foo - __per_cpu_start, %rax > mov %gs:per_cpu__foo - __per_cpu_start(%rip), %rax > > the last two lines will not assemble: > > t.S:5: Error: can't resolve `per_cpu__foo' {*UND* section} - > `__per_cpu_start' {*UND* section} > t.S:6: Error: can't resolve `per_cpu__foo' {*UND* section} - > `__per_cpu_start' {*UND* section} > > Unfortunately, the only way I can think of fixing this is to compute the > offset into a temp register, then use that: > > lea per_cpu__foo(%rip), %rax > mov %gs:__per_cpu_offset(%rax), %rax > > (where __per_cpu_offset is defined in the linker script as > -__per_cpu_start). > > This seems to be a general problem with zero-offset per-cpu. And its > unfortunate, because no-register access to per-cpu variables is nice to > have. > > The other alternative - and I have no idea whether this is practical or > possible - is to define a complete set of pre-offset per_cpu symbols. > > J
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