Re: [PATCH 2/2] i387: support lazy restore of FPU state

["H. Peter Anvin" <hpa@xxxxxxxxx>]

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.

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