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

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Date:   Mon, 09 Mar 2020 20:05:18 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Andy Lutomirski <luto@...nel.org>,
        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

Andy Lutomirski <luto@...nel.org> writes:
> On Mon, Mar 9, 2020 at 2:09 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>> Paolo Bonzini <pbonzini@...hat.com> writes:
>> > 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.
>>
>
> I'm okay with the save/restore dance, I guess.  It's just yet more
> entry crud to deal with architecture nastiness, except that this
> nastiness is 100% software and isn't Intel/AMD's fault.

And we can do it in C and don't have to fiddle with it in the ASM
maze.

Thanks,

        tglx