KVM: PPC: Book3S HV Nested: Avoid extra mftb() in nested entry

mftb() is expensive and one can be avoided on nested guest dispatch.

If the time checking code distinguishes between the L0 timer and the
nested HV timer, then both can be tested in the same place with the
same mftb() value.

This also nicely illustrates the relationship between the L0 and nested
HV timers.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211123095231.1036501-45-npiggin@gmail.com

arch/powerpc/include/asm/kvm_asm.h

@@ -79,6 +79,7 @@
 #define BOOK3S_INTERRUPT_FP_UNAVAIL	0x800
 #define BOOK3S_INTERRUPT_DECREMENTER	0x900
 #define BOOK3S_INTERRUPT_HV_DECREMENTER	0x980
+#define BOOK3S_INTERRUPT_NESTED_HV_DECREMENTER	0x1980
 #define BOOK3S_INTERRUPT_DOORBELL	0xa00
 #define BOOK3S_INTERRUPT_SYSCALL	0xc00
 #define BOOK3S_INTERRUPT_TRACE		0xd00

arch/powerpc/kvm/book3s_hv.c

@@ -1486,6 +1486,10 @@ static int kvmppc_handle_exit_hv(struct kvm_vcpu *vcpu,
 	run->ready_for_interrupt_injection = 1;
 	switch (vcpu->arch.trap) {
 	/* We're good on these - the host merely wanted to get our attention */
+	case BOOK3S_INTERRUPT_NESTED_HV_DECREMENTER:
+		WARN_ON_ONCE(1); /* Should never happen */
+		vcpu->arch.trap = BOOK3S_INTERRUPT_HV_DECREMENTER;
+		fallthrough;
 	case BOOK3S_INTERRUPT_HV_DECREMENTER:
 		vcpu->stat.dec_exits++;
 		r = RESUME_GUEST;
@@ -1814,6 +1818,12 @@ static int kvmppc_handle_nested_exit(struct kvm_vcpu *vcpu)
 		vcpu->stat.ext_intr_exits++;
 		r = RESUME_GUEST;
 		break;
+	/* These need to go to the nested HV */
+	case BOOK3S_INTERRUPT_NESTED_HV_DECREMENTER:
+		vcpu->arch.trap = BOOK3S_INTERRUPT_HV_DECREMENTER;
+		vcpu->stat.dec_exits++;
+		r = RESUME_HOST;
+		break;
 	/* SR/HMI/PMI are HV interrupts that host has handled. Resume guest.*/
 	case BOOK3S_INTERRUPT_HMI:
 	case BOOK3S_INTERRUPT_PERFMON:
@@ -3993,6 +4003,8 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
 		return BOOK3S_INTERRUPT_HV_DECREMENTER;
 	if (next_timer < time_limit)
 		time_limit = next_timer;
+	else if (*tb >= time_limit) /* nested time limit */
+		return BOOK3S_INTERRUPT_NESTED_HV_DECREMENTER;
 
 	vcpu->arch.ceded = 0;
 

arch/powerpc/kvm/book3s_hv_nested.c

@@ -375,11 +375,6 @@ long kvmhv_enter_nested_guest(struct kvm_vcpu *vcpu)
 	vcpu->arch.ret = RESUME_GUEST;
 	vcpu->arch.trap = 0;
 	do {
-		if (mftb() >= hdec_exp) {
-			vcpu->arch.trap = BOOK3S_INTERRUPT_HV_DECREMENTER;
-			r = RESUME_HOST;
-			break;
-		}
 		r = kvmhv_run_single_vcpu(vcpu, hdec_exp, lpcr);
 	} while (is_kvmppc_resume_guest(r));