linux-kernel - Re: [PATCH 3/3] x86, FPU: Do not use static_cpu

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Date:	Mon, 29 Apr 2013 08:42:30 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	X86 ML <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>,
	Borislav Petkov <bp@...e.de>
Subject: Re: [PATCH 3/3] x86, FPU: Do not use static_cpu_has before alternatives

On 04/29/2013 07:04 AM, Borislav Petkov wrote:
> From: Borislav Petkov <bp@...e.de>
> 
> The call stack below shows how this happens: basically eager_fpu_init()
> calls __thread_fpu_begin(current) which then does if (!use_eager_fpu()),
> which, in turn, uses static_cpu_has.
> 
> And we're executing before alternatives so static_cpu_has doesn't work
> there yet.
> 
> However, I don't want to cause the penalty of not using static_cpu_has
> to all callers of use_eager_fpu() which are a bunch spread around the
> tree so let's add the change only to __thread_fpu_begin() - it is still
> on the context switch path but we can at least keep the static_cpu_has
> call in the rest of the use_eager_fpu() callsites.
> 

I *was* considering adding static_cpu_has_safe() at some point which
would have a three-state jump, with the default (pre-alternatives) jump
pointing to dynamic detection code.

This might be useful here, on the other hand, perhaps it is acceptable
for use_eager_fpu() to be initially false?

	-hpa


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