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Date:   Tue, 28 Apr 2020 15:59:49 -0700
From:   Sean Christopherson <sean.j.christopherson@intel.com>
To:     Jim Mattson <jmattson@google.com>
Cc:     Paolo Bonzini <pbonzini@redhat.com>,
        Vitaly Kuznetsov <vkuznets@redhat.com>,
        Wanpeng Li <wanpengli@tencent.com>,
        Joerg Roedel <joro@8bytes.org>, kvm list <kvm@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Oliver Upton <oupton@google.com>,
        Peter Shier <pshier@google.com>
Subject: Re: [PATCH 09/13] KVM: nVMX: Prioritize SMI over nested IRQ/NMI
Message-ID: <20200428225949.GP12735@linux.intel.com>
References: <20200423022550.15113-1-sean.j.christopherson@intel.com>
 <20200423022550.15113-10-sean.j.christopherson@intel.com>
 <CALMp9eSuYqeVmWhb6q7T5DAW_Npbuin_N1+sbWjvcu0zTqiwsQ@mail.gmail.com>
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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@intel.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@intel.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.
Preemption timer is lower priority than MTF, ergo it's not in the same
class as SMI.

Regarding SMI vs. MTF and #DB trap, to actually prioritize SMIs above MTF
and #DBs, we'd need to save/restore MTF and pending #DBs via SMRAM.  I
think it makes sense to take the easy road and keep SMI after the traps,
with a comment to say it's technically wrong but not worth fixing.