KVM: PPC: Book3S HV P9: Reduce mftb per guest entry/exit

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.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Reviewed-by: Fabiano Rosas <farosas@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211123095231.1036501-7-npiggin@gmail.com

diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index 3322edb..5fc0c16 100644 (file)
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -3940,7 +3940,7 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
         *
         * XXX: Another day's problem.
         */
-       mtspr(SPRN_DEC, vcpu->arch.dec_expires - mftb());
+       mtspr(SPRN_DEC, vcpu->arch.dec_expires - tb);
 
        if (kvmhv_on_pseries()) {
                /*
@@ -4063,7 +4063,7 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, u64 time_limit,
        vc->in_guest = 0;
 
        next_timer = timer_get_next_tb();
-       set_dec(next_timer - mftb());
+       set_dec(next_timer - tb);
        /* We may have raced with new irq work */
        if (test_irq_work_pending())
                set_dec(1);

diff --git a/arch/powerpc/kvm/book3s_hv_p9_entry.c b/arch/powerpc/kvm/book3s_hv_p9_entry.c
index 0ff9ddb..bd8cf0a 100644 (file)
--- a/arch/powerpc/kvm/book3s_hv_p9_entry.c
+++ b/arch/powerpc/kvm/book3s_hv_p9_entry.c
@@ -203,7 +203,8 @@ int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpc
        unsigned long host_dawr1;
        unsigned long host_dawrx1;
 
-       hdec = time_limit - mftb();
+       tb = mftb();
+       hdec = time_limit - tb;
        if (hdec < 0)
                return BOOK3S_INTERRUPT_HV_DECREMENTER;
 
@@ -215,7 +216,7 @@ int kvmhv_vcpu_entry_p9(struct kvm_vcpu *vcpu, u64 time_limit, unsigned long lpc
        vcpu->arch.ceded = 0;
 
        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))