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

[Sean Christopherson <seanjc@google.com>]

On Thu, Sep 14, 2023, Like Xu wrote:
> On 2/9/2023 2:56 am, Jim Mattson wrote:
> > 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
> 
> As you said, that shouldn't be a problem.

It's still a bug though, overflow should only happen once.

> > 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.
> 
> Any obstruction issues on fixing in this direction ?

No, patch 2/2 in this series fixes LVTPC masking bug.  I haven't dug through all
of this yet, but my gut reaction is that I'm very strongly in favor of not using
irq_work just to ensure KVM kicks a vCPU out of HLT.  That's just ridiculous.