linux-kernel - Re: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly disable irqs

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Date:	Fri, 6 Mar 2015 17:31:06 +0100
From:	Oleg Nesterov <oleg@...hat.com>
To:	David Vrabel <david.vrabel@...rix.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Dave Hansen <dave.hansen@...el.com>,
	Borislav Petkov <bp@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Pekka Riikonen <priikone@....fi>,
	Rik van Riel <riel@...hat.com>,
	Suresh Siddha <sbsiddha@...il.com>,
	LKML <linux-kernel@...r.kernel.org>,
	"Yu, Fenghua" <fenghua.yu@...el.com>,
	Quentin Casasnovas <quentin.casasnovas@...cle.com>
Subject: Re: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly
	disable irqs

On 03/06, David Vrabel wrote:
>
> On 06/03/15 15:36, Oleg Nesterov wrote:
> >
> > This needs more discussion, but in short so far I think that fpu_alloc()
> > from #NM exception is fine if user_mode(regs) == T.
>
> I think a memory allocation here, where the only behaviour on a failure
> is to kill the task, is (and has always been) a crazy idea.

Well, I do not agree. But lets discuss this later. This code should be
rewritten in any case. It has more problems.

> Additionally, in a Xen PV guest the #NM handler is called with TS
> already cleared by the hypervisor so the handler must not enable
> interrupts (and thus potentially schedule another task) until after the
> current task's fpu state has been restored.  If a task was scheduled
> before restoring the FPU state, TS would be clear and that task will use
> fpu state from a previous task.

I can be easily wrong (especially because I know nothing about Xen ;), but
I do not think this is true.

Yes sure, we need to avoid preemption, but we need this in any case, even
without Xen.

Again, lets discuss this a bit later?

Oleg.

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