KVM: x86 shared msr infrastructure

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>

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 0558ff8..26a74b7 100644 (file)
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -809,4 +809,7 @@ 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 (file)
--- 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 e16cdc9..58c5cdd 100644 (file)
--- 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;
@@ -4815,6 +4893,9 @@ int kvm_arch_hardware_enable(void *garbage)
                int cpu = raw_smp_processor_id();
                per_cpu(cpu_tsc_khz, cpu) = 0;
        }
+
+       kvm_shared_msr_cpu_online();
+
        return kvm_x86_ops->hardware_enable(garbage);
 }