[PATCH v6 26/48] KVM: PPC: Book3S HV P9: Reduce mftb per guest entry/exit

[Nicholas Piggin]

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 <faro...@linux.ibm.com>
Signed-off-by: Nicholas Piggin <npig...@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 5c4ccebce682..f4e5a64457e6 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -3557,12 +3557,13 @@ 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))
@@ -3760,7 +3761,7 @@ 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()) {
                /*
@@ -3895,7 +3896,7 @@ 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