Date: Wed, 17 Jan 2007 00:25:14 +0100 From: "Attilio Rao" <attilio@freebsd.org> To: "Ivan Voras" <ivoras@fer.hr> Cc: freebsd-current@freebsd.org, freebsd-arch@freebsd.org Subject: Re: [PATCH] Mantaining turnstile aligned to 128 bytes in i386 CPUs Message-ID: <3bbf2fe10701161525j6ad9292y93502b8df0f67aa9@mail.gmail.com> In-Reply-To: <eojlnb$qje$1@sea.gmane.org> References: <3bbf2fe10607250813w8ff9e34pc505bf290e71758@mail.gmail.com> <3bbf2fe10607251004wf94e238xb5ea7a31c973817f@mail.gmail.com> <3bbf2fe10607261127p3f01a6c3w80027754f7d4e594@mail.gmail.com> <3bbf2fe10607281004o6727e976h19ee7e054876f914@mail.gmail.com> <3bbf2fe10701160851r79b04464m2cbdbb7f644b22b6@mail.gmail.com> <20070116154258.568e1aaf@pleiades.nextvenue.com> <b1fa29170701161355lc021b90o35fa5f9acb5749d@mail.gmail.com> <eoji7s$cit$2@sea.gmane.org> <b1fa29170701161425n7bcfe1e5m1b8c671caf3758db@mail.gmail.com> <eojlnb$qje$1@sea.gmane.org>
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2007/1/17, Ivan Voras <ivoras@fer.hr>: > Kip Macy wrote: > > On 1/16/07, Ivan Voras <ivoras@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@, 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
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