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

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Date:   Wed, 8 Apr 2020 00:04:39 +0200
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Thomas Gleixner <tglx@...utronix.de>,
        Andy Lutomirski <luto@...capital.net>,
        Vivek Goyal <vgoyal@...hat.com>
Cc:     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 07/04/20 22:20, Thomas Gleixner wrote:
>>> Havind said that, I thought disabling interrupts does not mask exceptions.
>>> So page fault exception should have been delivered even with interrupts
>>> disabled. Is that correct? May be there was no vm exit/entry during
>>> those 10 seconds and that's why.
> No. Async PF is not a real exception. It has interrupt semantics and it
> can only be injected when the guest has interrupts enabled. It's bad
> design.

Page-ready async PF has interrupt semantics.

Page-not-present async PF however does not have interrupt semantics, it
has to be injected immediately or not at all (falling back to host page
fault in the latter case).  So page-not-present async PF definitely
needs to be an exception, this is independent of whether it can be
injected when IF=0.

Hypervisors do not have any reserved exception vector, and must use
vectors up to 31, which is why I believe #PF was used in the first place
(though that predates my involvement in KVM by a few years).  These
days, #VE would be a much better exception to use instead (and it also
has a defined mechanism to avoid reentrancy).

Paolo