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

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Date:   Mon, 09 Mar 2020 10:09:00 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Paolo Bonzini <pbonzini@...hat.com>,
        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

Paolo Bonzini <pbonzini@...hat.com> writes:
> On 09/03/20 07:57, Thomas Gleixner wrote:
>> Thomas Gleixner <tglx@...utronix.de> writes:
>>
>> 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.

I'm fine with doing that save/restore dance, but I have no strong
opinion either.

> 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.

Memory store is not the problem. The real problem is hijacking #PF.

If you'd have just used a separate VECTOR_ASYNC_PF then none of these
problems would exist at all.

Thanks,

        tglx