[PATCH v5 26/48] KVM: PPC: Book3S HV P9: Reduce mftb per guest entry/exit

[Nicholas Piggin <npiggin@gmail.com>]

mftb is serialising (dispatch next-to-complete) so it is heavy weight
for a mfspr. Avoid reading it multiple times in the entry or exit paths.
A small number of cycles delay to timers is tolerable.

Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
---
 arch/powerpc/kvm/book3s_hv.c | 9 +++++----
 1 file changed, 5 insertions(+), 4 deletions(-)

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index dc1232d2a198..46f457c3b828 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -3551,12 +3551,13 @@ static int kvmhv_load_hv_regs_and_go(struct kvm_vcpu *vcpu, u64 time_limit,
 		host_dawrx1 = mfspr(SPRN_DAWRX1);
 	}
 
-	hdec = time_limit - mftb();
+	tb = mftb();
+	hdec = time_limit - tb;
 	if (hdec < 0)
 		return BOOK3S_INTERRUPT_HV_DECREMENTER;
 
 	if (vc->tb_offset) {
-		u64 new_tb = mftb() + vc->tb_offset;
+		u64 new_tb = tb + vc->tb_offset;
 		mtspr(SPRN_TBU40, new_tb);
 		tb = mftb();
 		if ((tb & 0xffffff) < (new_tb & 0xffffff))
@@ -3754,7 +3755,7 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
 	if (!(vcpu->arch.ctrl & 1))
 		mtspr(SPRN_CTRLT, mfspr(SPRN_CTRLF) & ~1);
 
-	mtspr(SPRN_DEC, vcpu->arch.dec_expires - mftb());
+	mtspr(SPRN_DEC, vcpu->arch.dec_expires - tb);
 
 	if (kvmhv_on_pseries()) {
 		/*
@@ -3889,7 +3890,7 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
 	vc->in_guest = 0;
 
 	next_timer = timer_get_next_tb();
-	mtspr(SPRN_DEC, next_timer - mftb());
+	mtspr(SPRN_DEC, next_timer - tb);
 	mtspr(SPRN_SPRG_VDSO_WRITE, local_paca->sprg_vdso);
 
 	kvmhv_load_host_pmu();
-- 
2.23.0