lkml.org 
[lkml]   [2008]   [Jun]   [4]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH 3/4] x86_64: Fold pda into per cpu area
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]
>

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


\
 
 \ /
  Last update: 2008-06-04 15:03    [W:0.270 / U:0.308 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site