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Date: Fri, 6 Mar 2015 17:31:06 +0100
From: Oleg Nesterov <oleg@redhat.com>
To: David Vrabel <david.vrabel@citrix.com>
Cc: Ingo Molnar <mingo@kernel.org>, Dave Hansen <dave.hansen@intel.com>,
        Borislav Petkov <bp@suse.de>, Andy Lutomirski <luto@amacapital.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
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        Suresh Siddha <sbsiddha@gmail.com>,
        LKML <linux-kernel@vger.kernel.org>,
        "Yu, Fenghua" <fenghua.yu@intel.com>,
        Quentin Casasnovas <quentin.casasnovas@oracle.com>
Subject: Re: [PATCH 1/1] x86/fpu: math_state_restore() should not blindly
	disable irqs
Message-ID: <20150306163106.GA30423@redhat.com>
References: <20150305195149.GB12657@redhat.com> <20150305201101.GA21571@gmail.com> <20150305212532.GA16890@redhat.com> <20150306075833.GA623@gmail.com> <20150306132634.GA20693@redhat.com> <20150306134601.GA11718@gmail.com> <20150306140154.GA22811@redhat.com> <54F9C112.3010604@citrix.com> <20150306153622.GA26939@redhat.com> <54F9D2A9.8030401@citrix.com>
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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.