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Subject: Re: [PATCH v2 2/5] x86/fpu: Fix FNSAVE usage in eagerfpu mode
To: Andy Lutomirski <luto@kernel.org>, x86@kernel.org,
        linux-kernel@vger.kernel.org
References: <cover.1453675014.git.luto@kernel.org>
 <60662444e13c76f06e23c15c5dcdba31b4ac3d67.1453675014.git.luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>, Fenghua Yu <fenghua.yu@intel.com>,
        Oleg Nesterov <oleg@redhat.com>, Peter Zijlstra <peterz@infradead.org>,
        Sai Praneeth Prakhya <sai.praneeth.prakhya@intel.com>,
        yu-cheng yu <yu-cheng.yu@intel.com>, Rik van Riel <riel@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <56A641FC.1030302@linux.intel.com>
Date: Mon, 25 Jan 2016 07:40:44 -0800
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On 01/24/2016 02:38 PM, Andy Lutomirski wrote:
>  	if (fpu->fpregs_active) {
> +		/*
> +		 * Ignore return value -- we don't care if reg state
> +		 * is clobbered.
> +		 */
>  		copy_fpregs_to_fpstate(fpu);
>  	} else {
>  		this_cpu_write(fpu_fpregs_owner_ctx, NULL);
> @@ -189,8 +193,12 @@ void fpu__save(struct fpu *fpu)
>  
>  	preempt_disable();
>  	if (fpu->fpregs_active) {
> -		if (!copy_fpregs_to_fpstate(fpu))
> -			fpregs_deactivate(fpu);
> +		if (!copy_fpregs_to_fpstate(fpu)) {
> +			if (use_eager_fpu())
> +				copy_kernel_to_fpregs(&fpu->state);
> +			else
> +				fpregs_deactivate(fpu);
> +		}
>  	}
>  	preempt_enable();

I wonder if we should just make the

> +			if (use_eager_fpu())
> +				copy_kernel_to_fpregs(&fpu->state);
> +			else
> +				fpregs_deactivate(fpu);

behavior the default _inside_ copy_fpregs_to_fpstate(fpu).  We evidently
got it wrong in 2/3 of the call sites that needed it.  It ends up being
an optimization for FNSAVE (because it allows us to avoid an FRSTOR),
but we only take advantage of that in cases of kernel_fpu_begin/end().

FXSAVE has been around since at _least_ 1999, and I'd expect it to get
used in place of FNSAVE everywhere that it is available.

If we don't want to do that, maybe we should add a "clobber" argument to
copy_fpregs_to_fpstate() for when it's allowed to clobber the register
state.

I just hate putting this logic at all the call sites.