Re: [PATCH 4/4] x86/fpu: don't abuse drop_init_fpu() in flush_thread()

From: Borislav Petkov
Date: Fri Mar 13 2015 - 12:21:32 EST

On Fri, Mar 13, 2015 at 03:55:42PM +0100, Oleg Nesterov wrote:
> But please look at drop_init_fpu(). If eagerfpu == F it calls drop_fpu() and
> this is what we need. flush_thread() already has the "if (!use_eager_fpu())",
> we can shift drop_fpu() there.
> Otherwise, if eagerfpu == T, drop_init_fpu() does restore_init_xstate() and
> this just burns CPU. Until flush_thread user_has_fpu/used_math this state
> restore_init_xstate() is pointless, this state will be lost after preemption.

Yeah, I was wondering why that's there.

One example where drop_init_fpu() seems to make sense is
__kernel_fpu_end(): kernel is done with FPU and current was using the
FPU prior so let's restore it for the eagerfpu case.



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