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

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Date:   Mon, 6 Apr 2020 13:32:01 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Vivek Goyal <vgoyal@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...nel.org>,
        Paolo Bonzini <pbonzini@...hat.com>,
        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 Apr 6, 2020, at 1:25 PM, Peter Zijlstra <peterz@...radead.org> wrote:
> 
> On Mon, Apr 06, 2020 at 03:09:51PM -0400, Vivek Goyal wrote:
>>> On Mon, Mar 09, 2020 at 09:22:15PM +0100, Peter Zijlstra wrote:
>>> On Mon, Mar 09, 2020 at 08:05:18PM +0100, Thomas Gleixner wrote:
>>>> Andy Lutomirski <luto@...nel.org> writes:
>>> 
>>>>> 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.
>>> 
>>> Right; I'd still love to kill KVM_ASYNC_PF_SEND_ALWAYS though, even if
>>> we do the save/restore in do_nmi(). That is some wild brain melt. Also,
>>> AFAIK none of the distros are actually shipping a PREEMPT=y kernel
>>> anyway, so killing it shouldn't matter much.
>> 
>> It will be nice if we can retain KVM_ASYNC_PF_SEND_ALWAYS. I have another
>> use case outside CONFIG_PREEMPT.
>> 
>> I am trying to extend async pf interface to also report page fault errors
>> to the guest.
> 
> Then please start over and design a sane ParaVirt Fault interface. The
> current one is utter crap.

Agreed. Don’t extend the current mechanism. Replace it.

I would be happy to review a replacement. I’m not really excited to review an extension of the current mess.  The current thing is barely, if at all, correct.