linux-kernel - [PATCH] KVM: x86: make KVM_REQ_NMI request iff NMI pending for vcpu

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Date: Wed,  3 Jan 2024 13:23:43 +0530
From: Prasad Pandit <ppandit@...hat.com>
To: Sean Christopherson <seanjc@...gle.com>
Cc: kvm@...r.kernel.org,
	linux-kernel@...r.kernel.org,
	Prasad Pandit <pjp@...oraproject.org>
Subject: [PATCH] KVM: x86: make KVM_REQ_NMI request iff NMI pending for vcpu

From: Prasad Pandit <pjp@...oraproject.org>

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@...oraproject.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