linux-kernel - Re: [patch 3/3] x86/fpu: Make FPU protection more robust

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Date:   Tue, 3 May 2022 11:06:22 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     Borislav Petkov <bp@...en8.de>,
        LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Filipe Manana <fdmanana@...e.com>
Subject: Re: [patch 3/3] x86/fpu: Make FPU protection more robust

On Mon, May 02, 2022 at 05:58:40PM +0200, Thomas Gleixner wrote:

> >> +void fpregs_lock(void)
> >> +{
> >> +	if (!IS_ENABLED(CONFIG_PREEMPT_RT))
> >> +		local_bh_disable();
> >> +	else
> >> +		preempt_disable();
> >
> > So I'm wondering: can we get rid of this distinction and simply do
> > preempt_disable()?
> > 
> > Or can FPU be used in softirq processing too so we want to block that
> > there?
> 
> Yes, FPU can be used legitimately in softirq processing context.
> 
> > But even if, fpu_in_use will already state that fact...
> 
> Right, though currently it's guaranteed that softirq processing context
> can use the FPU. Quite some of the network crypto work runs in softirq
> context, so this might cause a regression. If so, then this needs to be
> an explicit commit on top which is easy to revert. Let me stare at it
> some more.

Right, so with the:

	preempt_disable();
	this_cpu_write(fpu_in_use, true);
	barrier();

sequence it is safe against both softirq and hardirq fpu usage. The only
concern is performance not correctness when dropping that
local_bh_disable() thing.

So what Thomas proposes makes sense to me.