[PATCH 3/4] KVM: x86 shared msr infrastructure

Avi Kivity
2009-09-16T12:45:59Z

The various syscall-related MSRs are fairly expensive to switch. Currently
we switch them on every vcpu preemption, which is far too often:

- if we're switching to a kernel thread (idle task, threaded interrupt,
kernel-mode virtio server (vhost-net), for example) and back, then
there's no need to switch those MSRs since kernel threasd won't
be exiting to userspace.

- if we're switching to another guest running an identical OS, most likely
those MSRs will have the same value, so there's little point in reloading
them.

- if we're running the same OS on the guest and host, the MSRs will have
identical values and reloading is unnecessary.

This patch uses the new user return notifiers to implement last-minute
switching, and checks the msr values to avoid unnecessary reloading.

Signed-off-by: Avi Kivity <avi@redhat.com>
---
arch/x86/include/asm/kvm_host.h | 3 +
arch/x86/kvm/Kconfig | 1 +
arch/x86/kvm/x86.c | 79 +++++++++++++++++++++++++++++++++++++++
3 files changed, 83 insertions(+), 0 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 45226f0..863bde8 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -799,4 +799,7 @@ int kvm_cpu_has_interrupt(struct kvm_vcpu *vcpu);
int kvm_arch_interrupt_allowed(struct kvm_vcpu *vcpu);
int kvm_cpu_get_interrupt(struct kvm_vcpu *v);

+void kvm_define_shared_msr(unsigned index, u32 msr);
+void kvm_set_shared_msr(unsigned index, u64 val);
+
#endif /* _ASM_X86_KVM_HOST_H */
diff --git a/arch/x86/kvm/Kconfig b/arch/x86/kvm/Kconfig
index b84e571..4cd4983 100644
--- a/arch/x86/kvm/Kconfig
+++ b/arch/x86/kvm/Kconfig
@@ -28,6 +28,7 @@ config KVM
select HAVE_KVM_IRQCHIP
select HAVE_KVM_EVENTFD
select KVM_APIC_ARCHITECTURE
+ select USER_RETURN_NOTIFIER
---help---
Support hosting fully virtualized guest machines using hardware
virtualization extensions. You will need a fairly recent
diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
index 1d454d9..0e7c40c 100644
--- a/arch/x86/kvm/x86.c
+++ b/arch/x86/kvm/x86.c
@@ -37,6 +37,7 @@
#include <linux/iommu.h>
#include <linux/intel-iommu.h>
#include <linux/cpufreq.h>
+#include <linux/user-return-notifier.h>
#include <trace/events/kvm.h>
#undef TRACE_INCLUDE_FILE
#define CREATE_TRACE_POINTS
@@ -87,6 +88,25 @@ EXPORT_SYMBOL_GPL(kvm_x86_ops);
int ignore_msrs = 0;
module_param_named(ignore_msrs, ignore_msrs, bool, S_IRUGO | S_IWUSR);

+#define KVM_NR_SHARED_MSRS 16
+
+struct kvm_shared_msrs_global {
+ int nr;
+ struct kvm_shared_msr {
+ u32 msr;
+ u64 value;
+ } msrs[KVM_NR_SHARED_MSRS];
+};
+
+struct kvm_shared_msrs {
+ struct user_return_notifier urn;
+ bool registered;
+ u64 current_value[KVM_NR_SHARED_MSRS];
+};
+
+static struct kvm_shared_msrs_global __read_mostly shared_msrs_global;
+static DEFINE_PER_CPU(struct kvm_shared_msrs, shared_msrs);
+
struct kvm_stats_debugfs_item debugfs_entries[] = {
{ "pf_fixed", VCPU_STAT(pf_fixed) },
{ "pf_guest", VCPU_STAT(pf_guest) },
@@ -123,6 +143,64 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
{ NULL }
};

+static void kvm_on_user_return(struct user_return_notifier *urn)
+{
+ unsigned slot;
+ struct kvm_shared_msr *global;
+ struct kvm_shared_msrs *locals
+ = container_of(urn, struct kvm_shared_msrs, urn);
+
+ for (slot = 0; slot < shared_msrs_global.nr; ++slot) {
+ global = &shared_msrs_global.msrs[slot];
+ if (global->value != locals->current_value[slot]) {
+ wrmsrl(global->msr, global->value);
+ locals->current_value[slot] = global->value;
+ }
+ }
+ locals->registered = false;
+ user_return_notifier_unregister(urn);
+}
+
+void kvm_define_shared_msr(unsigned slot, u32 msr)
+{
+ int cpu;
+ u64 value;
+
+ if (slot >= shared_msrs_global.nr)
+ shared_msrs_global.nr = slot + 1;
+ shared_msrs_global.msrs[slot].msr = msr;
+ rdmsrl_safe(msr, &value);
+ shared_msrs_global.msrs[slot].value = value;
+ for_each_online_cpu(cpu)
+ per_cpu(shared_msrs, cpu).current_value[slot] = value;
+}
+EXPORT_SYMBOL_GPL(kvm_define_shared_msr);
+
+static void kvm_shared_msr_cpu_online(void)
+{
+ unsigned i;
+ struct kvm_shared_msrs *locals = &__get_cpu_var(shared_msrs);
+
+ for (i = 0; i < shared_msrs_global.nr; ++i)
+ locals->current_value[i] = shared_msrs_global.msrs[i].value;
+}
+
+void kvm_set_shared_msr(unsigned slot, u64 value)
+{
+ struct kvm_shared_msrs *smsr = &__get_cpu_var(shared_msrs);
+
+ if (value == smsr->current_value[slot])
+ return;
+ smsr->current_value[slot] = value;
+ wrmsrl(shared_msrs_global.msrs[slot].msr, value);
+ if (!smsr->registered) {
+ smsr->urn.on_user_return = kvm_on_user_return;
+ user_return_notifier_register(&smsr->urn);
+ smsr->registered = true;
+ }
+}
+EXPORT_SYMBOL_GPL(kvm_set_shared_msr);
+
unsigned long segment_base(u16 selector)
{
struct descriptor_table gdt;
@@ -4696,6 +4774,7 @@ int kvm_arch_vcpu_reset(struct kvm_vcpu *vcpu)

void kvm_arch_hardware_enable(void *garbage)
{
+ kvm_shared_msr_cpu_online();
kvm_x86_ops->hardware_enable(garbage);
}

--
1.6.4.1