linux-kernel - Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND

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Date:   Sat, 07 Mar 2020 20:01:04 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Andy Lutomirski <luto@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

Andy Lutomirski <luto@...nel.org> writes:
> On Sat, Mar 7, 2020 at 7:47 AM Thomas Gleixner <tglx@...utronix.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.

Thanks,

        tglx