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From: Thomas Gleixner <tglx@linutronix.de>
To: Andy Lutomirski <luto@kernel.org>
Cc: LKML <linux-kernel@vger.kernel.org>, X86 ML <x86@kernel.org>,
	kvm list <kvm@vger.kernel.org>,
	Paolo Bonzini <pbonzini@redhat.com>,
	stable <stable@vger.kernel.org>
Subject: Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS
Date: Mon, 09 Mar 2020 07:57:47 +0100	[thread overview]
Message-ID: <877dzu8178.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <87wo7v8g4j.fsf@nanos.tec.linutronix.de>

Thomas Gleixner <tglx@linutronix.de> writes:

> Thomas Gleixner <tglx@linutronix.de> writes:
>> Andy Lutomirski <luto@kernel.org> writes:
>>> On Sat, Mar 7, 2020 at 7:47 AM Thomas Gleixner <tglx@linutronix.de> wrote:
>>>> The host knows exactly when it injects a async PF and it can store CR2
>>>> and reason of that async PF in flight.
>>>>
>>>> On the next VMEXIT it checks whether apf_reason is 0. If apf_reason is 0
>>>> then it knows that the guest has read CR2 and apf_reason. All good
>>>> nothing to worry about.
>>>>
>>>> If not it needs to be careful.
>>>>
>>>> As long as the apf_reason of the last async #PF is not cleared by the
>>>> guest no new async #PF can be injected. That's already correct because
>>>> in that case IF==0 which prevents a nested async #PF.
>>>>
>>>> If MCE, NMI trigger a real pagefault then the #PF injection needs to
>>>> clear apf_reason and set the correct CR2. When that #PF returns then the
>>>> old CR2 and apf_reason need to be restored.
>>>
>>> How is the host supposed to know when the #PF returns?  Intercepting
>>> IRET sounds like a bad idea and, in any case, is not actually a
>>> reliable indication that #PF returned.
>>
>> The host does not care about the IRET. It solely has to check whether
>> apf_reason is 0 or not. That way it knows that the guest has read CR2
>> and apf_reason.
>
> Bah. I'm a moron. Of course it needs to trap the IRET of the #PF in
> order to restore CR2 and apf_reason. Alternatively it could trap the CR2
> read of #PF, but yes that's all nasty.

Some hours or sleep and not staring at this meess later and while
reading the leaves of my morning tea:

guest side:

   nmi()/mce() ...
   
        stash_crs();

+       stash_and_clear_apf_reason();

        ....

+       restore_apf_reason();

	restore_cr2();

Too obvious, isn't it?

Thanks,

        tglx

  reply	other threads:[~2020-03-09  6:57 UTC|newest]

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-03-07  2:26 [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS Andy Lutomirski
2020-03-07 15:03 ` Andy Lutomirski
2020-03-07 15:47   ` Thomas Gleixner
2020-03-07 15:59     ` Andy Lutomirski
2020-03-07 19:01       ` Thomas Gleixner
2020-03-07 19:34         ` Andy Lutomirski
2020-03-08  7:23         ` Thomas Gleixner
2020-03-09  6:57           ` Thomas Gleixner [this message]
2020-03-09  8:40             ` Paolo Bonzini
2020-03-09  9:09               ` Thomas Gleixner
2020-03-09 18:14                 ` Andy Lutomirski
2020-03-09 19:05                   ` Thomas Gleixner
2020-03-09 20:22                     ` Peter Zijlstra
2020-04-06 19:09                       ` Vivek Goyal
2020-04-06 20:25                         ` Peter Zijlstra
2020-04-06 20:32                           ` Andy Lutomirski
2020-04-06 20:42                             ` Andy Lutomirski
2020-04-07 17:21                               ` Vivek Goyal
2020-04-07 17:38                                 ` Andy Lutomirski
2020-04-07 20:20                                   ` Thomas Gleixner
2020-04-07 21:41                                     ` Andy Lutomirski
2020-04-07 22:07                                       ` Paolo Bonzini
2020-04-07 22:29                                         ` Andy Lutomirski
2020-04-08  0:30                                           ` Paolo Bonzini
2020-05-21 15:55                                         ` Vivek Goyal
2020-04-07 22:48                                       ` Thomas Gleixner
2020-04-08  4:48                                         ` Andy Lutomirski
2020-04-08  9:32                                           ` Borislav Petkov
2020-04-08 10:12                                           ` Thomas Gleixner
2020-04-08 18:23                                           ` Vivek Goyal
2020-04-07 22:49                                       ` Vivek Goyal
2020-04-08 10:01                                         ` Borislav Petkov
2020-04-07 22:04                                     ` Paolo Bonzini
2020-04-07 23:21                                       ` Thomas Gleixner
2020-04-08  8:23                                         ` Paolo Bonzini
2020-04-08 13:01                                           ` Thomas Gleixner
2020-04-08 15:38                                             ` Peter Zijlstra
2020-04-08 16:41                                               ` Thomas Gleixner
2020-04-09  9:03                                             ` Paolo Bonzini
2020-04-08 15:34                                           ` Sean Christopherson
2020-04-08 16:50                                             ` Paolo Bonzini
2020-04-08 18:01                                               ` Thomas Gleixner
2020-04-08 20:34                                                 ` Vivek Goyal
2020-04-08 23:06                                                   ` Thomas Gleixner
2020-04-08 23:14                                                     ` Thomas Gleixner
2020-04-09  4:50                                                 ` Andy Lutomirski
2020-04-09  9:43                                                   ` Paolo Bonzini
2020-04-09 11:36                                                   ` Andrew Cooper
2020-04-09 12:47                                                   ` Paolo Bonzini
2020-04-09 14:13                                                     ` Andrew Cooper
2020-04-09 14:32                                                       ` Paolo Bonzini
2020-04-09 15:03                                                         ` Andy Lutomirski
2020-04-09 15:17                                                           ` Paolo Bonzini
2020-04-09 17:32                                                             ` Andy Lutomirski
2020-04-06 21:32                         ` Thomas Gleixner

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