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

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Date:   Mon, 9 Mar 2020 09:40:11 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, X86 ML <x86@...nel.org>,
        kvm list <kvm@...r.kernel.org>, stable <stable@...r.kernel.org>
Subject: Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS

On 09/03/20 07:57, Thomas Gleixner wrote:
> Thomas Gleixner <tglx@...utronix.de> writes:
>> Thomas Gleixner <tglx@...utronix.de> writes:
>>> Andy Lutomirski <luto@...nel.org> writes:
>>>> On Sat, Mar 7, 2020 at 7:47 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>>>>> 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.
>>>>
>>> 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.
> 
> 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?

Yes, this works but Andy was not happy about adding more
save-and-restore to NMIs.  If you do not want to do that, I'm okay with
disabling async page fault support for now.

Storing the page fault reason in memory was not a good idea.  Better
options would be to co-opt the page fault error code (e.g. store the
reason in bits 31:16, mark bits 15:0 with the invalid error code
RSVD=1/P=0), or to use the virtualization exception area.

Paolo