linux-kernel - Re: [PATCH 09/13] KVM: nVMX: Prioritize SMI over nested IRQ/NMI

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Message-ID: <CALMp9eR8VsGAPrKLM2ZCG_DBTJa5Cn+raaUtzG=4V-bp_jxqWw@mail.gmail.com>
Date:   Tue, 28 Apr 2020 16:23:52 -0700
From:   Jim Mattson <jmattson@...gle.com>
To:     Sean Christopherson <sean.j.christopherson@...el.com>
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Vitaly Kuznetsov <vkuznets@...hat.com>,
        Wanpeng Li <wanpengli@...cent.com>,
        Joerg Roedel <joro@...tes.org>, kvm list <kvm@...r.kernel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Oliver Upton <oupton@...gle.com>,
        Peter Shier <pshier@...gle.com>
Subject: Re: [PATCH 09/13] KVM: nVMX: Prioritize SMI over nested IRQ/NMI

On Tue, Apr 28, 2020 at 3:59 PM Sean Christopherson
<sean.j.christopherson@...el.com> wrote:
>
> On Tue, Apr 28, 2020 at 03:04:02PM -0700, Jim Mattson wrote:
> > On Wed, Apr 22, 2020 at 7:26 PM Sean Christopherson
> > <sean.j.christopherson@...el.com> wrote:
> > >
> > > Check for an unblocked SMI in vmx_check_nested_events() so that pending
> > > SMIs are correctly prioritized over IRQs and NMIs when the latter events
> > > will trigger VM-Exit.  This also fixes an issue where an SMI that was
> > > marked pending while processing a nested VM-Enter wouldn't trigger an
> > > immediate exit, i.e. would be incorrectly delayed until L2 happened to
> > > take a VM-Exit.
> > >
> > > Fixes: 64d6067057d96 ("KVM: x86: stubs for SMM support")
> > > Signed-off-by: Sean Christopherson <sean.j.christopherson@...el.com>
> > > ---
> > >  arch/x86/kvm/vmx/nested.c | 6 ++++++
> > >  1 file changed, 6 insertions(+)
> > >
> > > diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> > > index 1fdaca5fd93d..8c16b190816b 100644
> > > --- a/arch/x86/kvm/vmx/nested.c
> > > +++ b/arch/x86/kvm/vmx/nested.c
> > > @@ -3750,6 +3750,12 @@ static int vmx_check_nested_events(struct kvm_vcpu *vcpu)
> > >                 return 0;
> > >         }
> > >
> > > +       if (vcpu->arch.smi_pending && !is_smm(vcpu)) {
> > > +               if (block_nested_events)
> > > +                       return -EBUSY;
> > > +               goto no_vmexit;
> > > +       }
> > > +
> >
> > From the SDM, volume 3:
> >
> > • System-management interrupts (SMIs), INIT signals, and higher
> > priority events take priority over MTF VM exits.
> >
> > I think this block needs to be moved up.
>
> Hrm.  It definitely needs to be moved above the preemption timer, though I
> can't find any public documentation about the preemption timer's priority.

Section 25.2 of the SDM, volume 3:

Debug-trap exceptions and higher priority events take priority over VM
exits caused by the VMX-preemption
timer. VM exits caused by the VMX-preemption timer take priority over
VM exits caused by the “NMI-window
exiting” VM-execution control and lower priority events.