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

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Date:   Tue, 7 Apr 2020 15:29:23 -0700
From:   Andy Lutomirski <luto@...capital.net>
To:     Paolo Bonzini <pbonzini@...hat.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Vivek Goyal <vgoyal@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        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 Apr 7, 2020, at 3:07 PM, Paolo Bonzini <pbonzini@...hat.com> wrote:
> 
> On 07/04/20 23:41, Andy Lutomirski wrote:
>> 2. Access to bad memory results in #MC.  Sure, #MC is a turd, but
>> it’s an *architectural* turd. By all means, have a nice simple PV
>> mechanism to tell the #MC code exactly what went wrong, but keep the
>> overall flow the same as in the native case.
>> 
>> I think I like #2 much better. It has another nice effect: a good
>> implementation will serve as a way to exercise the #MC code without
>> needing to muck with EINJ or with whatever magic Tony uses. The
>> average kernel developer does not have access to a box with testable
>> memory failure reporting.
> 
> I prefer #VE, but I can see how #MC has some appeal.  However, #VE has a
> mechanism to avoid reentrancy, unlike #MC.  How would that be better
> than the current mess with an NMI happening in the first few
> instructions of the #PF handler?
> 
> 

It has to be an IST vector due to the possibility of hitting a memory failure right after SYSCALL.  I imagine that making #VE use IST would be unfortunate.

I think #MC has a mechanism to prevent reentrancy to a limited extent. How does #VE avoid reentrancy?