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

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Message-ID: <20200406202505.GO2452@worktop.programming.kicks-ass.net>
Date:   Mon, 6 Apr 2020 22:25:05 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Vivek Goyal <vgoyal@...hat.com>
Cc:     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 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.