Re: [PATCH v2 3/3] i387: support lazy restore of FPU state

[Josh Boyer]

On Mon, Feb 20, 2012 at 2:48 PM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
> Date: Sun, 19 Feb 2012 13:27:00 -0800
> Subject: [PATCH v2 3/3] i387: support lazy restore of FPU state
>
> This makes us recognize when we try to restore FPU state that matches
> what we already have in the FPU on this CPU, and avoids the restore
> entirely if so.
>
> To do this, we add two new data fields:
>
>  - a percpu 'fpu_owner_task' variable that gets written any time we
>   update the "has_fpu" field, and thus acts as a kind of back-pointer
>   to the task that owns the CPU.  The exception is when we save the FPU
>   state as part of a context switch - if the save can keep the FPU
>   state around, we leave the 'fpu_owner_task' variable pointing at the
>   task whose FP state still remains on the CPU.
>
>  - a per-thread 'last_cpu' field, that indicates which CPU that thread
>   used its FPU on last.  We update this on every context switch
>   (writing an invalid CPU number if the last context switch didn't
>   leave the FPU in a lazily usable state), so we know that *that*
>   thread has done nothing else with the FPU since.
>
> These two fields together can be used when next switching back to the
> task to see if the CPU still matches: if 'fpu_owner_task' matches the
> task we are switching to, we know that no other task (or kernel FPU
> usage) touched the FPU on this CPU in the meantime, and if the current
> CPU number matches the 'last_cpu' field, we know that this thread did no
> other FP work on any other CPU, so the FPU state on the CPU must match
> what was saved on last context switch.
>
> In that case, we can avoid the 'f[x]rstor' entirely, and just clear the
> CR0.TS bit.
>
> Signed-off-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>

I haven't tried really figuring this out yet, but building the Fedora kernel
on x86_64 with your latest tree results in:

ERROR: "fpu_owner_task" [lib/raid6/raid6_pq.ko] undefined!
ERROR: "fpu_owner_task" [arch/x86/kvm/kvm.ko] undefined!
ERROR: "fpu_owner_task" [arch/x86/crypto/sha1-ssse3.ko] undefined!
ERROR: "fpu_owner_task" [arch/x86/crypto/serpent-sse2-x86_64.ko] undefined!
ERROR: "fpu_owner_task" [arch/x86/crypto/ghash-clmulni-intel.ko] undefined!
make[1]: *** [__modpost] Error 1
make: *** [modules] Error 2
+ exit 1

Since this patch went in as 7e16838d94b566a1, I'm guessing it's at least
related.

I'm building again with more verbose output but I thought I'd send this out
quickly.

josh
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