Re: [PATCH 1/2] KVM: x86: Synthesize at most one PMI per VM-exit

[Sean Christopherson <seanjc@google.com>]

On Mon, Sep 25, 2023, Mingwei Zhang wrote:
> On Mon, Sep 25, 2023 at 10:59 AM Sean Christopherson <seanjc@google.com> wrote:
> >
> > On Mon, Sep 25, 2023, Mingwei Zhang wrote:
> > > From: Jim Mattson <jmattson@google.com>
> > >
> > > When the irq_work callback, kvm_pmi_trigger_fn(), is invoked during a
> > > VM-exit that also invokes __kvm_perf_overflow() as a result of
> > > instruction emulation, kvm_pmu_deliver_pmi() will be called twice
> > > before the next VM-entry.
> > >
> > > That shouldn't be a problem. The local APIC is supposed to
> > > automatically set the mask flag in LVTPC when it handles a PMI, so the
> > > second PMI should be inhibited. However, KVM's local APIC emulation
> > > fails to set the mask flag in LVTPC when it handles a PMI, so two PMIs
> > > are delivered via the local APIC. In the common case, where LVTPC is
> > > configured to deliver an NMI, the first NMI is vectored through the
> > > guest IDT, and the second one is held pending. When the NMI handler
> > > returns, the second NMI is vectored through the IDT. For Linux guests,
> > > this results in the "dazed and confused" spurious NMI message.
> > >
> > > Though the obvious fix is to set the mask flag in LVTPC when handling
> > > a PMI, KVM's logic around synthesizing a PMI is unnecessarily
> > > convoluted.
> >
> > Unless Jim outright objects, I strongly prefer placing this patch second, with
> > the above two paragraphs replaced with my suggestion (or something similar):
> >
> >   Calling kvm_pmu_deliver_pmi() twice is unlikely to be problematic now that
> >   KVM sets the LVTPC mask bit when delivering a PMI.  But using IRQ work to
> >   trigger the PMI is still broken, albeit very theoretically.
> >
> >   E.g. if the self-IPI to trigger IRQ work is be delayed long enough for the
> >   vCPU to be migrated to a different pCPU, then it's possible for
> >   kvm_pmi_trigger_fn() to race with the kvm_pmu_deliver_pmi() from
> >   KVM_REQ_PMI and still generate two PMIs.
> >
> >   KVM could set the mask bit using an atomic operation, but that'd just be
> >   piling on unnecessary code to workaround what is effectively a hack.  The
> >   *only* reason KVM uses IRQ work is to ensure the PMI is treated as a wake
> >   event, e.g. if the vCPU just executed HLT.
> >
> > I understand Jim's desire for the patch to be more obviously valuable, but the
> > people that need convincing are already convinced that the patch is worth taking.
> >
> > > Remove the irq_work callback for synthesizing a PMI, and all of the
> > > logic for invoking it. Instead, to prevent a vcpu from leaving C0 with
> > > a PMI pending, add a check for KVM_REQ_PMI to kvm_vcpu_has_events().
> > >
> > > Fixes: 9cd803d496e7 ("KVM: x86: Update vPMCs when retiring instructions")
> > > Signed-off-by: Jim Mattson <jmattson@google.com>
> > > Tested-by: Mingwei Zhang <mizhang@google.com>
> > > Tested-by: Dapeng Mi <dapeng1.mi@linux.intel.com>
> >
> > Needs your SoB
> 
> Signed-off-by: Mingwei Zhang <mizhang@google.com>

Thanks!

Jim gave his blessing off-list for swapping the order, I'll do that and massage
the changelogs when applying, i.e. no need for a v3.