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[Qemu-ppc] [PATCH 17/22] ppc: Avoid decrementer related kvm exits

From: Alexander Graf
Subject: [Qemu-ppc] [PATCH 17/22] ppc: Avoid decrementer related kvm exits
Date: Sun, 30 Oct 2011 21:23:08 +0100

From: David Gibson <address@hidden>

In __cpu_ppc_store_decr(), we set up a regular timer used to trigger
decrementer interrupts.  This is necessary to implement the decrementer
properly under TCG, but is unnecessary under KVM (true for both Book3S-PR
and Book3S-HV KVM variants), because the kernel handles generating and
delivering decrementer exceptions.

Under kvm, in fact, the timer causes expensive and unnecessary exits from
kvm to qemu.  This patch, therefore, disables setting the timer when kvm
is in use.

Signed-off-by: Anton Blanchard <address@hidden>
Signed-off-by: David Gibson <address@hidden>
Signed-off-by: Alexander Graf <address@hidden>
 hw/ppc.c |    6 ++++++
 1 files changed, 6 insertions(+), 0 deletions(-)

diff --git a/hw/ppc.c b/hw/ppc.c
index 25b59dd..d29af0b 100644
--- a/hw/ppc.c
+++ b/hw/ppc.c
@@ -662,6 +662,12 @@ static void __cpu_ppc_store_decr (CPUState *env, uint64_t 
     LOG_TB("%s: %08" PRIx32 " => %08" PRIx32 "\n", __func__,
                 decr, value);
+    if (kvm_enabled()) {
+        /* KVM handles decrementer exceptions, we don't need our own timer */
+        return;
+    }
     now = qemu_get_clock_ns(vm_clock);
     next = now + muldiv64(value, get_ticks_per_sec(), tb_env->decr_freq);
     if (is_excp) {

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