Re: About patch bdedff263132 - KVM: x86: Route pending NMIs

[Prasad Pandit]

Hello Sean,

On Mon, 30 Oct 2023 at 20:41, Sean Christopherson <seanjc@xxxxxxxxxx> wrote:
>> if a fix patch like below would be acceptable OR reverting above patch is
>> more reasonable?
>
> No, a revert would break AMD's vNMI.

* Okay, that confirmation helps.

>> -               kvm_make_request(KVM_REQ_NMI, vcpu);
>> +               if (events->nmi.pending)
>> +                       kvm_make_request(KVM_REQ_NMI, vcpu);
>
> This looks sane, but it should be unnecessary as KVM_REQ_NMI nmi_queued=0 should
> be a (costly) nop.  Hrm, unless the vCPU is in HLT, in which case KVM will treat
> a spurious KVM_REQ_NMI as a wake event.  When I made this change, my assumption
> was that userspace would set KVM_VCPUEVENT_VALID_NMI_PENDING iff there was
> relevant information to process.  But if I'm reading the code correctly, QEMU
> invokes KVM_SET_VCPU_EVENTS with KVM_VCPUEVENT_VALID_NMI_PENDING at the end of
> machine creation.
>
> Hmm, but even that should be benign unless userspace is stuffing other guest
> state.  E.g. KVM will spuriously exit to userspace with -EAGAIN while the vCPU
> is in KVM_MP_STATE_UNINITIALIZED, and I don't see a way for the vCPU to be put
> into a blocking state after transitioning out of UNINITIATED via INIT+SIPI without
> processing KVM_REQ_NMI.
>
> Please provide more information on what is breaking and/or how to reproduce the
> issue.  E.g. at the very least, a trace of KVM_{G,S}ET_VCPU_EVENTS.   There's not
> even enough info here to write a changelog.
>

* I see, I'll try to understand in more detail about what's really
happening and will get back asap.

Thank you.
---
  - Prasad