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Date: Sun, 19 Feb 2012 14:44:31 -0800 From: "H. Peter Anvin" <hpa@...or.com> To: Linus Torvalds <torvalds@...ux-foundation.org> CC: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, x86@...nel.org, Linux Kernel Mailing List <linux-kernel@...r.kernel.org> Subject: Re: [PATCH 2/2] i387: support lazy restore of FPU state On 02/19/2012 02:37 PM, Linus Torvalds wrote: > > - on *every* task switch from task A, we write A->thread.fpu.last_cpu, > whether we owned the FPU or not. And we only write a real CPU number in > the case where we owned it, and the FPU save left the state untouched > in the FPU. > > - so when we switch into task A next time, comparing the current CPU > number with that 'last_cpu' field inarguably says "when I last switched > out, I really saved it on this CPU" > > That, together with verifying that the per-cpu "fpu_owner_task" matches > "task A", guarantees that the state is really valid. Because we will > clear (or set to another task) fpu_owner_task if it ever gets > switched to anything else. > > But somebody should really validate this. Think through all the > kernel_fpu_begin() etc cases. I think it looks pretty obvious, and it > really does seem to work and improve task switching, but... > I think your logic is correct but suboptimal. What would make more sense to me is that we write last_cpu when we *load* the state. After all, if you didn't load the state you couldn't have modified it. In kernel_fpu_begin, *if* we end up flushing the state, we should set last_cpu to -1 indicating that *no* CPU currently owns the state -- after all, even on this CPU we would now have to reload the state from memory. Does that make sense? -hpa -- H. Peter Anvin, Intel Open Source Technology Center I work for Intel. I don't speak on their behalf. -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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