[PATCH v2] KVM: PPC: Book3S HV: Avoid spurious interrupts caused by LPCR_MER bit

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 once LPCR_MER bit is set, it is supposed to be reset by
software. But when a vCPU starts running with LPCR_MER set, the vCPU does
not exit back to the host until the decrementer expires or there is an
hcall, etc. This is because KVM on PowerNV guests have support for
native XIVE, so they are not dependent on host for interrupt emulation.
Due to this behaviour, a huge number of spurious interrupts are seen
since LPCR_MER continues to be set and LPCR_MER cannot be reset until the
vCPU exits to the host.

Fix this behaviour by not using the LPCR_MER bit in case of KVM on
PowerNV, as the XIVE hardware can present interrupts to the KVM guest
vCPU directly. So the LPCR_MER functionality is not required.
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>
---
v2:
1. Handle the case where xive_interrupt_pending() is true and also the
external exception bit is set.

 arch/powerpc/kvm/book3s_hv.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index dbac3573b2c8..f4d12a97311b 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c

@@ -4980,7 +4980,7 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
 			if (!kvmhv_on_pseries() && (__kvmppc_get_msr_hv(vcpu) & MSR_EE))
 				kvmppc_inject_interrupt_hv(vcpu,
 							   BOOK3S_INTERRUPT_EXTERNAL, 0);
-			else
+			else if (!xics_on_xive())
 				lpcr |= LPCR_MER;
 		} else {
 			/*

-- 
2.55.0