[PATCH v4 25/46] KVM: PPC: Book3S HV P9: Use host timer accounting to avoid decrementer read

[Nicholas Piggin]

There is no need to save away the host DEC value, as it is derived
from the host timer subsystem, which maintains the next timer time.

Signed-off-by: Nicholas Piggin <npig...@gmail.com>
---
 arch/powerpc/include/asm/time.h |  5 +++++
 arch/powerpc/kernel/time.c      |  1 +
 arch/powerpc/kvm/book3s_hv.c    | 12 ++++++------
 3 files changed, 12 insertions(+), 6 deletions(-)

diff --git a/arch/powerpc/include/asm/time.h b/arch/powerpc/include/asm/time.h
index 68d94711811e..0128cd9769bc 100644
--- a/arch/powerpc/include/asm/time.h
+++ b/arch/powerpc/include/asm/time.h
@@ -101,6 +101,11 @@ extern void __init time_init(void);
 
 DECLARE_PER_CPU(u64, decrementers_next_tb);
 
+static inline u64 timer_get_next_tb(void)
+{
+       return __this_cpu_read(decrementers_next_tb);
+}
+
 /* Convert timebase ticks to nanoseconds */
 unsigned long long tb_to_ns(unsigned long long tb_ticks);
 
diff --git a/arch/powerpc/kernel/time.c b/arch/powerpc/kernel/time.c
index b67d93a609a2..c5d524622c17 100644
--- a/arch/powerpc/kernel/time.c
+++ b/arch/powerpc/kernel/time.c
@@ -108,6 +108,7 @@ struct clock_event_device decrementer_clockevent = {
 EXPORT_SYMBOL(decrementer_clockevent);
 
 DEFINE_PER_CPU(u64, decrementers_next_tb);
+EXPORT_SYMBOL_GPL(decrementers_next_tb);
 static DEFINE_PER_CPU(struct clock_event_device, decrementers);
 
 #define XSEC_PER_SEC (1024*1024)
diff --git a/arch/powerpc/kvm/book3s_hv.c b/arch/powerpc/kvm/book3s_hv.c
index bb30c5ab53d1..db807eebb3bd 100644
--- a/arch/powerpc/kvm/book3s_hv.c
+++ b/arch/powerpc/kvm/book3s_hv.c
@@ -3686,16 +3686,16 @@ static int kvmhv_p9_guest_entry(struct kvm_vcpu *vcpu, 
u64 time_limit,
        unsigned long host_amr = mfspr(SPRN_AMR);
        unsigned long host_fscr = mfspr(SPRN_FSCR);
        s64 dec;
-       u64 tb;
+       u64 tb, next_timer;
        int trap, save_pmu;
 
-       dec = mfspr(SPRN_DEC);
        tb = mftb();
-       if (dec < 0)
+       next_timer = timer_get_next_tb();
+       if (tb >= next_timer)
                return BOOK3S_INTERRUPT_HV_DECREMENTER;
-       local_paca->kvm_hstate.dec_expires = dec + tb;
-       if (local_paca->kvm_hstate.dec_expires < time_limit)
-               time_limit = local_paca->kvm_hstate.dec_expires;
+       local_paca->kvm_hstate.dec_expires = next_timer;
+       if (next_timer < time_limit)
+               time_limit = next_timer;
 
        vcpu->arch.ceded = 0;
 
-- 
2.23.0