Re: [PATCH] Mantaining turnstile aligned to 128 bytes in i386 CPUs

2007/1/17, Ivan Voras <ivoras_at_fer.hr>:
> Kip Macy wrote:
> > On 1/16/07, Ivan Voras <ivoras_at_fer.hr> wrote:
> >> But it does seem to hurt the performance a bit - maybe it's time to add
> >> another CPU option like I586_CPU and I686_CPU?
> >
> > Unless there is a compelling reason not to do so, I think that that
> > would be a good idea.
>
> Maybe even someone finds a way to get optimized versions of memcpy in
> the kernel :)
>
> I was thinking: AFAIK the only major stopper is context saving of the
> various "auxiliary" registers - FPU, MMX, SSE, right? But is it an
> all-or-nothing situation? I.e. does it make sense (can it be done?) to
> just elect to save the MMX context? (AFAIK they are different registers
> than SSE, but overlay FPU registers?) The idea is to save something
> smaller than the full set.

When I implemented fpu copy into the kernel I had a lot of thinking
about this and I think it is possible at least with some restrictions.
For example, for an xmm copy you would just save 8 registers content
but you  have to ensure no pending FPU exceptions will break your
kernel and so you should preserve a clean copy of FPU state or, treact
the corner cases you can get.
For xmm, after some very productive discussions with bde_at_, we arrived
at the conclusion that should be pretty safe to just have an 16 byte
aligned buffer for registers saving (in this way you can use 8 movdqa
for saving them) but I didn't end to play with it.
(My implementation should deal with the problem of pinning the
scheduler too, in order to avoid a wrong reading of per-cpu datas).

Attilio


-- 
Peace can only be achieved by understanding - A. Einstein