linux-kernel - Re: [PATCH 3/4] KVM: async_pf: Force a nested vmexit if the injected #PF is async

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Message-ID: <20170614132039.GC1276@potion>
Date:   Wed, 14 Jun 2017 15:20:40 +0200
From:   Radim Krčmář <rkrcmar@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>
Cc:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        kvm <kvm@...r.kernel.org>, Paolo Bonzini <pbonzini@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>
Subject: Re: [PATCH 3/4] KVM: async_pf: Force a nested vmexit if the injected
 #PF is async_pf

2017-06-14 21:02+0800, Wanpeng Li:
> 2017-06-14 20:52 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> > 2017-06-14 09:07+0800, Wanpeng Li:
> >> 2017-06-14 2:55 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> >> > Using vcpu->arch.cr2 is suspicious as VMX doesn't update CR2 on VM
> >> > exits;  isn't this going to change the CR2 visible in L2 guest after a
> >> > nested VM entry?
> >>
> >> Sorry, I don't fully understand the question. As you know this
> >> vcpu->arch.cr2 which includes token is set before async pf injection,
> >
> > Yes, I'm thinking that setting vcpu->arch.cr2 is a mistake in this case.
> >
> >> and L1 will intercept it from EXIT_QUALIFICATION during nested vmexit,
> >
> > Right, so we do not need to have the token in CR2, because L1 is not
> > going to look at it.
> >
> >> why it can change the CR2 visible in L2 guest after a nested VM entry?
> >
> > Sorry, the situation is too convoluted to be expressed in one sentence:
> >
> > 1) L2 is running with CR2 = L2CR2
> > 3) VMX exits (say, unrelated EXTERNAL_INTERRUPT) and L0 stores L2CR2 in
> >    vcpu->arch.cr2
> > 2) APF for L1 has completed
> > 4) L0 KVM wants to inject APF and sets vcpu->arch.cr2 = APFT
> > 5) L0 KVM does a nested VM exit to L1, EXIT_QUALIFICATION = APFT
> > 6) L0 KVM enters L1 with CR2 = vcpu->arch.cr2 = APFT
> > 7) L1 stores APFT as L2's CR2
> > 8) L1 handles APF, maybe reschedules, but eventually comes back to this
> >    L2's thread
> > 9) after some time, L1 enters L2 with CR2 = APFT
> > 10) L2 is running with CR2 = APTF
> >
> > The original L2CR2 is lost and we'd introduce a bug if L2 wanted to look
> > at it, e.g. it was in a process of handling its #PF.
> 
> Good point. What's your proposal? :)

Get rid of async_pf. :) Optimal solutions aside, I think it would be
best to add a new injection function for APF.  One that injects a normal
#PF for non-nested guests and directly triggers a #PF VM exit otherwise,
and call it from kvm_arch_async_page_*present().

Do you think that just moving the nested VM exit from
nested_vmx_check_exception() would work?

Thanks.