Re: [patch 00/41] cpu alloc / cpu ops v3: Optimize per cpu access

Eric Dumazet
2008-06-09T20:16:39Z

Andi Kleen a écrit :
> Christoph Lameter <clameter@sgi.com> writes:
>
>> The problem is that offsets relative to %gs or %fs are limited by the
>> small memory model that is chosen.
>
> Actually they are not. If you really want you can do
> movabs $64bit,%reg ; op ...,%gs:(%reg)
> It's just not very efficient compared to small (or rather kernel) model
> and also older binutils didn't support large model.
>

I am not sure Christoph was refering to actual instructions.

I was suggesting using for static percpu (vmlinux or modules) :

vmlinux : (offset31 computed by linker at vmlinux link edit time)
incl %gs:offset31

modules : (offset31 computed at module load time by module loader)
incl %gs:offset31

(If we make sure all this stuff is allocated in first chunk)

And for dynamic percpu :

movq field(%rdi),%rax
incl %gs:(%rax) /* full 64bits 'offsets' */

I understood (but might be wrong again) that %gs itself could not be used with an offset > 2GB, because
the way %gs segment is setup. So in the 'dynamic percpu' case, %rax should not exceed 2^31