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From: Dave Hansen <dave.hansen@linux.intel.com>
To: Andy Lutomirski <luto@kernel.org>,
	x86@kernel.org, linux-kernel@vger.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>
Subject: Re: [PATCH v2 2/5] x86/fpu: Fix FNSAVE usage in eagerfpu mode
Date: Mon, 25 Jan 2016 07:40:44 -0800	[thread overview]
Message-ID: <56A641FC.1030302@linux.intel.com> (raw)
In-Reply-To: <60662444e13c76f06e23c15c5dcdba31b4ac3d67.1453675014.git.luto@kernel.org>

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.

  reply	other threads:[~2016-01-25 15:40 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2016-01-24 22:38 [PATCH v2 0/5] x86/fpu: eagerfpu fixes, speedups, and default enablement Andy Lutomirski
2016-01-24 22:38 ` [PATCH v2 1/5] x86/fpu: Fix math emulation in eager fpu mode Andy Lutomirski
2016-02-09 16:10   ` [tip:x86/fpu] " tip-bot for Andy Lutomirski
2016-01-24 22:38 ` [PATCH v2 2/5] x86/fpu: Fix FNSAVE usage in eagerfpu mode Andy Lutomirski
2016-01-25 15:40   ` Dave Hansen [this message]
2016-01-25 17:25     ` Andy Lutomirski
2016-01-25 17:26       ` Dave Hansen
2016-02-09 16:10   ` [tip:x86/fpu] " tip-bot for Andy Lutomirski
2016-01-24 22:38 ` [PATCH v2 3/5] x86/fpu: Fold fpu_copy into fpu__copy Andy Lutomirski
2016-02-09 16:10   ` [tip:x86/fpu] x86/fpu: Fold fpu_copy() into fpu__copy() tip-bot for Andy Lutomirski
2016-01-24 22:38 ` [PATCH v2 4/5] x86/fpu: Speed up lazy FPU restores slightly Andy Lutomirski
2016-02-09 16:11   ` [tip:x86/fpu] " tip-bot for Andy Lutomirski
2016-01-24 22:38 ` [PATCH v2 5/5] x86/fpu: Default eagerfpu=on on all CPUs Andy Lutomirski
2016-02-09 16:11   ` [tip:x86/fpu] " tip-bot for Andy Lutomirski

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