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Date: Mon, 8 Jan 2024 09:38:03 -0800
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Subject: Re: [PATCH] KVM: x86: make KVM_REQ_NMI request iff NMI pending for vcpu
From: Sean Christopherson <seanjc@google.com>
To: Prasad Pandit <ppandit@redhat.com>
Cc: kvm@vger.kernel.org, linux-kernel@vger.kernel.org, 
	Prasad Pandit <pjp@fedoraproject.org>
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On Mon, Jan 08, 2024, Prasad Pandit wrote:
> On Wed, 3 Jan 2024 at 13:24, Prasad Pandit <ppandit@redhat.com> wrote:
> > kvm_vcpu_ioctl_x86_set_vcpu_events() routine makes 'KVM_REQ_NMI'
> > request for a vcpu even when its 'events->nmi.pending' is zero.
> > Ex:
> >     qemu_thread_start
> >      kvm_vcpu_thread_fn
> >       qemu_wait_io_event
> >        qemu_wait_io_event_common
> >         process_queued_cpu_work
> >          do_kvm_cpu_synchronize_post_init/_reset
> >           kvm_arch_put_registers
> >            kvm_put_vcpu_events (cpu, level=[2|3])
> >
> > This leads vCPU threads in QEMU to constantly acquire & release the
> > global mutex lock, delaying the guest boot due to lock contention.
> > Add check to make KVM_REQ_NMI request only if vcpu has NMI pending.
> >
> > Fixes: bdedff263132 ("KVM: x86: Route pending NMIs from userspace through process_nmi()")
> > Signed-off-by: Prasad Pandit <pjp@fedoraproject.org>
> > ---
> >  arch/x86/kvm/x86.c | 3 ++-
> >  1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> > index 1a3aaa7dafae..468870450b8b 100644
> > --- a/arch/x86/kvm/x86.c
> > +++ b/arch/x86/kvm/x86.c
> > @@ -5405,7 +5405,8 @@ static int kvm_vcpu_ioctl_x86_set_vcpu_events(struct kvm_vcpu *vcpu,
> >         if (events->flags & KVM_VCPUEVENT_VALID_NMI_PENDING) {
> >                 vcpu->arch.nmi_pending = 0;
> >                 atomic_set(&vcpu->arch.nmi_queued, events->nmi.pending);
> > -               kvm_make_request(KVM_REQ_NMI, vcpu);
> > +               if (events->nmi.pending)
> > +                       kvm_make_request(KVM_REQ_NMI, vcpu);
> >         }
> >         static_call(kvm_x86_set_nmi_mask)(vcpu, events->nmi.masked);
> > --
> > 2.43.0
> 
> Ping...!

This is on my list of things to grab for 6.8, I'm just waiting for various pull
requests to fully land in order to simplify my branch management.