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

2007/1/17, Maxim Sobolev <sobomax@xxxxxxxxxxx>:
Attilio Rao wrote:
> 2007/1/17, Ivan Voras <ivoras@xxxxxx>:
>> Kip Macy wrote:
>> > On 1/16/07, Ivan Voras <ivoras@xxxxxx> 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@, 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).

I might be wrong, but I think the DragonFly has solved this issue (i.e.
optimized memcpy in the kernel) somehow quite some time ago.

Dragonfly saves the whole context (xmm + mmx + fpu state). It is a too
heavy mechanism ATM for us (and for them too I suspect). The don't
need to deal with pinning too, BTW.

Attilio


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