Date: Fri, 28 Mar 2014 19:36:31 -0600 From: Chris Torek <torek@torek.net> To: Konstantin Belousov <kostikbel@gmail.com> Cc: freebsd-hackers@freebsd.org Subject: Re: slight problem with r254457 (kthread_add fix) Message-ID: <201403290136.s2T1aVT6078723@elf.torek.net> In-Reply-To: Your message of "Fri, 28 Mar 2014 16:24:02 %2B0200." <20140328142402.GX21331@kib.kiev.ua>
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>The kernel-FPU code only exists for x86. I'm a bit surprised kernel-mode FPU is not supported on sparc64, but that does seem to be the case. (Surprising since there *is* code to use the VIS block-copy instructions, which use the FPU registers. This seems to be handled as a *very* special case though: it is called only through cpu_block_copy, and only for pmap_copy_page, which cannot be re-entered. So it's OK, if suboptimal. Using the VIS block-copy for other large copies should be a performance win, but this requires the ability to re-enter the routine.) Anyway, I'm mostly just saying that I can't check the remaining architectures as I don't know them well enough to tell. If you do, all is well there. :-) >Did you tested the patch I posted, for your situation ? It took a while to get back to, but yes, it works fine. [On cpu_throw:] >It seems that you formulation somewhat contradictory. The cpu_throw >is used only to do the last switch out of the exiting thread. Ah, I was thinking of the code paths that get there by calling sched_throw() with NULL, in the various mp_machdep.c files. You're looking at the one other code path that reaches sched_throw() (with a non-NULL argument), in thread_exit(): >And indeed, I think that there is a bug, on x86. The CR0.TS must be >cleared, and fpcurthread must be reset if current cpu still carries >FPU state of the curthread. At least, I do not see anything which >would guarantee that CLTS was done before cpu_throw. It turns out this is OK because of this: cpu_thread_exit(td); /* XXXSMP */ cpu_thread_exit() does an fpudrop if needed. (I don't think this is a particularly clean design, but it does work.) Chris
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