[PATCH 2/3] Add a vcpu_migrated statistic

Leonard Norrgard
2007-03-02T16:53:16Z

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The previous patch introduces an IPI on every vcpu migration on SMP
machines to find out the TSC value on the previous cpu. We might be
able to avoid this IPI on systems that have a synchronized TSC by
checking for it in svm/vmx_vcpu_load:

+ /* If this isn't the first vcpu_load and if the TSC
+ doesn't run at a constant rate, we must handle TSC
+ offsets. */
+ if (vcpu->cpu != -1
+ && !boot_cpu_has(X86_FEATURE_CONSTANT_TSC)) {

Wether this is actually reliable and working in the intended way is
not know yet. In the meantime, introduce a statistic for vcpu
migration, so we get some decision support for the needs for such an
optimization.

Signed-off-by: Leonard Norrgård <vinsci-g2GXA8XeJSExHbG02/KK1g@public.gmane.org>

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Index: linux-2.6/drivers/kvm/kvm_main.c
===================================================================
--- linux-2.6.orig/drivers/kvm/kvm_main.c 2007-03-02 17:31:59.000000000 +0200
+++ linux-2.6/drivers/kvm/kvm_main.c 2007-03-02 17:34:13.000000000 +0200
@@ -67,6 +67,7 @@
{ "halt_exits", &kvm_stat.halt_exits },
{ "request_irq", &kvm_stat.request_irq_exits },
{ "irq_exits", &kvm_stat.irq_exits },
+ { "vcpu_migrated", &kvm_stat.vcpu_migrated },
{ NULL, NULL }
};

Index: linux-2.6/drivers/kvm/svm.c
===================================================================
--- linux-2.6.orig/drivers/kvm/svm.c 2007-03-02 17:32:02.000000000 +0200
+++ linux-2.6/drivers/kvm/svm.c 2007-03-02 17:33:13.000000000 +0200
@@ -621,6 +621,8 @@
if (vcpu->cpu != -1) {
u64 tsc_this, tsc_previous;

+ ++kvm_stat.vcpu_migrated;
+
/* Get TSC value for this and the previous cpu. */
rdtscll(tsc_this);
smp_call_function_single(vcpu->cpu, ipi_rdtscll,
Index: linux-2.6/drivers/kvm/vmx.c
===================================================================
--- linux-2.6.orig/drivers/kvm/vmx.c 2007-03-02 17:32:06.000000000 +0200
+++ linux-2.6/drivers/kvm/vmx.c 2007-03-02 17:33:34.000000000 +0200
@@ -242,6 +242,8 @@
if (vcpu->cpu != -1) {
u64 tsc_this, tsc_previous, guest_tsc_offset;

+ ++kvm_stat.vcpu_migrated;
+
/* Get TSC value for this and the previous cpu. */
rdtscll(tsc_this);
smp_call_function_single(vcpu->cpu, ipi_rdtscll,

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