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

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Date:   Mon, 9 Mar 2020 11:14:04 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Thomas Gleixner <tglx@...utronix.de>
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

On Mon, Mar 9, 2020 at 2:09 AM Thomas Gleixner <tglx@...utronix.de> wrote:
>
> 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.
>

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.

At least until we get an async page fault due to apf_reason being paged out...