[PATCH] KVM: PPC: Book3S HV: Avoid triggering an extra interrupt for a vCPU

A huge number of spurious interrupts can be seen immediately after a KVM
on PowerNV guest boots up in XIVE mode.

$ cat /proc/interrupts  | grep SPU
SPU:     223705     192439     273526     147623   Spurious interrupts

This bug was introduced by commit ecd10702baae5 ("KVM: PPC: Book3S HV:
Handle pending exceptions on guest entry with MSR_EE"). The root cause
is that when there is an interrupt pending for a vCPU
(xive_interrupt_pending() returns true) and MSR_EE is disabled for the
vCPU, LPCR_MER ends up getting set for the VCPU. When the vCPU starts
running, the XIVE hardware presents the pending interrupt to the vCPU
(since the KVM guest has native XIVE support) and then a second spurious
interrupt gets presented to the vCPU due to the LPCR_MER bit being set.

Fix this behaviour by not queuing up any extra interrupts with LPCR_MER
if there is an interrupt already pending in the case of KVM on PowerNV.
This reduces the number of spurious interrupts drastically.

Fixes: ecd10702baae5 ("KVM: PPC: Book3S HV: Handle pending exceptions on guest entry with MSR_EE")
Cc: stable@vger.kernel.org #6.8+
Reported-by: Timothy Pearson <tpearson@raptorengineering.com>
Closes: https://lore.kernel.org/linuxppc-dev/582904882.11159.1786719390349.JavaMail.zimbra@raptorengineeringinc.com (local)
Signed-off-by: Gautam Menghani <redacted>
---
 arch/powerpc/kvm/book3s_hv.c | 5 ++---
 1 file changed, 2 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 7667563fb9ff..fda3767ebc9e 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c

@@ -4937,9 +4937,8 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
 
 	if (!nested) {
 		kvmppc_core_prepare_to_enter(vcpu);
-		if (test_bit(BOOK3S_IRQPRIO_EXTERNAL,
-			     &vcpu->arch.pending_exceptions) ||
-		    xive_interrupt_pending(vcpu)) {
+		if (!xive_interrupt_pending(vcpu) && test_bit(BOOK3S_IRQPRIO_EXTERNAL,
+							      &vcpu->arch.pending_exceptions)) {
 			/*
 			 * For nested HV, don't synthesize but always pass MER,
 			 * the L0 will be able to optimise that more

-- 
2.55.0