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
next prev parent 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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