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Message-ID: <4FBBB4A0.7010500@redhat.com>
Date: Tue, 22 May 2012 18:45:36 +0300
From: Avi Kivity <avi@...hat.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
CC: Steven Rostedt <rostedt@...dmis.org>,
linux-kernel <linux-kernel@...r.kernel.org>,
Ingo Molnar <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
Thomas Gleixner <tglx@...utronix.de>,
Paul Turner <pjt@...gle.com>,
Peter Zijlstra <peterz@...radead.org>,
Frederic Weisbecker <fweisbec@...il.com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
Subject: Re: NMI vs #PF clash
On 05/22/2012 06:33 PM, Linus Torvalds wrote:
> On Tue, May 22, 2012 at 7:27 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
>>
>> Is reading it fast? Then we could do a two reads and only write when
>> needed.
>
> Even better: we could do nothing at all.
>
> We could just say: let's make sure that any #PF case that can happen
> in #NMI can also be re-done with arbitrary 'error_code' and 'struct
> regs' contents.
>
> At that point, what could happen is
> - #PF
> - NMI
> - #PF
> - read cr2 for NMI fault
> - handle the NMI #PF
> - return from #PF
> - return from #NMI
> - read cr2 for original #PF fault - but get the NMI cr2 again
> - hande the #PF again (this should be a no-op now)
> - return from #PF
> - instruction restart causes new #PF
> - now we do the original page fault
>
> So one option is to just make sure that the few cases (just the
> vmalloc area?) that NMI can trigger are all ok to be re-done with
> other state.
>
> I note that right now we have
>
> if (unlikely(fault_in_kernel_space(address))) {
> if (!(error_code & (PF_RSVD | PF_USER | PF_PROT))) {
> if (vmalloc_fault(address) >= 0)
> return;
>
> and that the error_code check means that the retried NMI #PF would not
> go through that. But maybe we don't even need that check?
>
> That error_code thing seems to literally be the only thing that keeps
> us from just re-doing the vmalloc_fault() silently.
>
do_page_fault() is not the only code that relies on cr2; vmx_vcpu_run()
is the other. If an NMI happens there, and takes a #PF, then the guest
will run with a bad cr2.
(svm saves and restores cr2 in microcode, and also provides a way to
mask NMIs, so it isn't vulnerable to this issue).
--
error compiling committee.c: too many arguments to function
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