KVM: PPC: Use clockevent multiplier and shifter for decrementer

Time for which the hrtimer is started for decrementer emulation is calculated
using tb_ticks_per_usec. While hrtimer uses the clockevent for DEC
reprogramming (if needed) and which calculate timebase ticks using the
multiplier and shifter mechanism implemented within clockevent layer.

It was observed that this conversion (timebase->time->timebase) are not
correct because the mechanism are not consistent.
In our setup it adds 2% jitter.

With this patch clockevent multiplier and shifter mechanism are used when
starting hrtimer for decrementer emulation. Now the jitter is < 0.5%.

Signed-off-by: Bharat Bhushan <bharat.bhushan@freescale.com>
Signed-off-by: Alexander Graf <agraf@suse.de>

1 files changed, 7 insertions, 2 deletions

diff --git a/arch/powerpc/kvm/emulate.c b/arch/powerpc/kvm/emulate.c
index afc9154f1aef..b5872f61a213 100644
--- a/arch/powerpc/kvm/emulate.c
+++ b/arch/powerpc/kvm/emulate.c
@@ -23,6 +23,7 @@
 #include <linux/types.h>
 #include <linux/string.h>
 #include <linux/kvm_host.h>
+#include <linux/clockchips.h>
 
 #include <asm/reg.h>
 #include <asm/time.h>
@@ -104,8 +105,12 @@ void kvmppc_emulate_dec(struct kvm_vcpu *vcpu)
          */
 
         dec_time = vcpu->arch.dec;
-        dec_time *= 1000;
-        do_div(dec_time, tb_ticks_per_usec);
+        /*
+         * Guest timebase ticks at the same frequency as host decrementer.
+         * So use the host decrementer calculations for decrementer emulation.
+         */
+        dec_time = dec_time << decrementer_clockevent.shift;
+        do_div(dec_time, decrementer_clockevent.mult);
         dec_nsec = do_div(dec_time, NSEC_PER_SEC);
         hrtimer_start(&vcpu->arch.dec_timer,
                 ktime_set(dec_time, dec_nsec), HRTIMER_MODE_REL);