KVM: PPC: Book3S HV: Handle pending exceptions on guest entry with MSR_EE

Commit 026728dc ("KVM: PPC: Book3S HV P9: Inject pending xive
interrupts at guest entry") changed guest entry so that if external
interrupts are enabled, BOOK3S_IRQPRIO_EXTERNAL is not tested for. Test
for this regardless of MSR_EE.

For an L1 host, do not inject an interrupt, but always
use LPCR_MER. If the L0 desires it can inject an interrupt.

Fixes: 026728dc ("KVM: PPC: Book3S HV P9: Inject pending xive interrupts at guest entry")
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
[jpn: use kvmpcc_get_msr(), write commit message]
Signed-off-by: Jordan Niethe <jniethe5@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20231201132618.555031-7-vaibhav@linux.ibm.com

arch/powerpc/kvm/book3s_hv.c

@@ -4738,13 +4738,19 @@ int kvmhv_run_single_vcpu(struct kvm_vcpu *vcpu, u64 time_limit,
 
 	if (!nested) {
 		kvmppc_core_prepare_to_enter(vcpu);
-		if (__kvmppc_get_msr_hv(vcpu) & MSR_EE) {
-			if (xive_interrupt_pending(vcpu))
+		if (test_bit(BOOK3S_IRQPRIO_EXTERNAL,
+			     &vcpu->arch.pending_exceptions) ||
+		    xive_interrupt_pending(vcpu)) {
+			/*
+			 * For nested HV, don't synthesize but always pass MER,
+			 * the L0 will be able to optimise that more
+			 * effectively than manipulating registers directly.
+			 */
+			if (!kvmhv_on_pseries() && (__kvmppc_get_msr_hv(vcpu) & MSR_EE))
 				kvmppc_inject_interrupt_hv(vcpu,
-						BOOK3S_INTERRUPT_EXTERNAL, 0);
-		} else if (test_bit(BOOK3S_IRQPRIO_EXTERNAL,
-			     &vcpu->arch.pending_exceptions)) {
-			lpcr |= LPCR_MER;
+							   BOOK3S_INTERRUPT_EXTERNAL, 0);
+			else
+				lpcr |= LPCR_MER;
 		}
 	} else if (vcpu->arch.pending_exceptions ||
 		   vcpu->arch.doorbell_request ||