linux-kernel - Re: [PATCH v3] KVM: nVMX: Fix attempting to emulate "Acknowledge interrupt on exit" when there is no interrupt which L1 requires to inject to L2

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Date:   Thu, 3 Aug 2017 14:30:20 +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 v3] KVM: nVMX: Fix attempting to emulate "Acknowledge
 interrupt on exit" when there is no interrupt which L1 requires to inject to
 L2

2017-08-03 07:01+0800, Wanpeng Li:
> 2017-08-03 4:26 GMT+08:00 Radim Krčmář <rkrcmar@...hat.com>:
> > 2017-08-02 03:48-0700, Wanpeng Li:
> >> From: Wanpeng Li <wanpeng.li@...mail.com>
> >> diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
> >> @@ -10761,7 +10761,8 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu, bool external_intr)
> >>               return 0;
> >>       }
> >>
> >> -     if ((kvm_cpu_has_interrupt(vcpu) || external_intr) &&
> >> +     if ((kvm_cpu_has_interrupt(vcpu) ||
> >> +         (external_intr && !nested_exit_intr_ack_set(vcpu))) &&
> >
> > I think it would be safer to also add something like the second hunk I
> > posted (that also takes nested_exit_on_intr() into account).
> >
> > The issue is that we're allowing L2's GUEST_RFLAGS and
> > GUEST_INTERRUPTIBILITY_INFO to disable userspace interrupt injection
> > even though neither affect delivery of interrupts into L1.
> > This means that L2 can block/postpone the delivery to L1 by doing "cli;
> > busy_loop/normal_critical_section".
> 
> Ouch! My fault, the v3 patch w/o the second hunk and w/ the second
> hunk both can result in L1 guest softlockup. I just tested the patch
> with L2 windows guest yesterday, however, the softlockup can happen
> when the L2 is the linux guest. So should we still take the v2 for the
> moment?

Sure, that one is an improvement over the current situation (I guess it
doesn't break any hypervisor).
I'll just add a comment about its incorrectness.