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Subject: Re: [PATCH v2] x86/kvm: Disable KVM_ASYNC_PF_SEND_ALWAYS
To:     Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Andrew Cooper <andrew.cooper3@citrix.com>
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From:   Paolo Bonzini <pbonzini@redhat.com>
Message-ID: <04aca08a-cfce-b4db-559a-23aee0a0b7aa@redhat.com>
Date:   Thu, 9 Apr 2020 14:47:10 +0200
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On 09/04/20 06:50, Andy Lutomirski wrote:
> The small
> (or maybe small) one is that any fancy protocol where the guest
> returns from an exception by doing, logically:
> 
> Hey I'm done;  /* MOV somewhere, hypercall, MOV to CR4, whatever */
> IRET;
> 
> is fundamentally racy.  After we say we're done and before IRET, we
> can be recursively reentered.  Hi, NMI!

That's possible in theory.  In practice there would be only two levels
of nesting, one for the original page being loaded and one for the tail
of the #VE handler.  The nested #VE would see IF=0, resolve the EPT
violation synchronously and both handlers would finish.  For the tail
page to be swapped out again, leading to more nesting, the host's LRU
must be seriously messed up.

With IST it would be much messier, and I haven't quite understood why
you believe the #VE handler should have an IST.

Anyhow, apart from the above "small" issue, we have these separate parts:

1) deliver page-ready notifications via interrupt

2) page-in hypercall + deliver page-not-found notifications via #VE

3) propagation of host-side SIGBUS

all of which have both a host and a guest part, and all of which make
(more or less) sense independent of the other.

Paolo