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Date: Fri, 6 Mar 2015 16:36:22 +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>,
        Pekka Riikonen <priikone@iki.fi>, Rik van Riel <riel@redhat.com>,
        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: <20150306153622.GA26939@redhat.com>
References: <54F74F59.5070107@intel.com> <20150305195127.GA12657@redhat.com> <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>
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On 03/06, David Vrabel wrote:
>
> On 06/03/15 14:01, Oleg Nesterov wrote:
>
> > Not sure I understand it correctly after the first quick look, but
> >
> > 1. It conflicts with the recent changes in tip/x86/fpu
> >
> > 2. fpu_ini() initializes current->thread.fpu.state. This looks unneeded,
> >    the kernel threads no longer have FPU context and do not abuse CPU.
> >
> > 3. I can be easily wrong, but it looks buggy... Note that
> >    arch_dup_task_struct() doesn't allocate child->fpu.state if
> >    !tsk_used_math(parent).
>
> ...yes. It's bit-rotted a bit.
>
> > No, I do not think this patch is a good idea. Perhaps I am wrong, but I
> > think we need other changes. And they should start from init_fpu().
>
> But the general principle of avoiding the allocation in the #NM handler
> and hence avoiding the need to re-enable IRQs is still a good idea, yes?

This needs more discussion, but in short so far I think that fpu_alloc()
from #NM exception is fine if user_mode(regs) == T.

Just do_device_not_available() should simply do conditional_sti(), I think.
Perhaps it can even enable irqs unconditionally, but we need to verify that
this is 100% correct.

And I agree that "if (!tsk_used_math(tsk))" code in math_state_restore()
should be removed. But not to avoid the allocation, and this needs other
changes.

Oleg.