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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>
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3 changed files with 85 additions and 0 deletions
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@ -809,4 +809,7 @@ int kvm_cpu_has_interrupt(struct kvm_vcpu *vcpu);
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int kvm_arch_interrupt_allowed(struct kvm_vcpu *vcpu);
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int kvm_cpu_get_interrupt(struct kvm_vcpu *v);
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void kvm_define_shared_msr(unsigned index, u32 msr);
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void kvm_set_shared_msr(unsigned index, u64 val);
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#endif /* _ASM_X86_KVM_HOST_H */
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@ -28,6 +28,7 @@ config KVM
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select HAVE_KVM_IRQCHIP
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select HAVE_KVM_EVENTFD
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select KVM_APIC_ARCHITECTURE
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select USER_RETURN_NOTIFIER
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---help---
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Support hosting fully virtualized guest machines using hardware
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virtualization extensions. You will need a fairly recent
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@ -37,6 +37,7 @@
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#include <linux/iommu.h>
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#include <linux/intel-iommu.h>
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#include <linux/cpufreq.h>
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#include <linux/user-return-notifier.h>
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#include <trace/events/kvm.h>
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#undef TRACE_INCLUDE_FILE
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#define CREATE_TRACE_POINTS
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@ -87,6 +88,25 @@ EXPORT_SYMBOL_GPL(kvm_x86_ops);
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int ignore_msrs = 0;
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module_param_named(ignore_msrs, ignore_msrs, bool, S_IRUGO | S_IWUSR);
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#define KVM_NR_SHARED_MSRS 16
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struct kvm_shared_msrs_global {
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int nr;
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struct kvm_shared_msr {
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u32 msr;
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u64 value;
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} msrs[KVM_NR_SHARED_MSRS];
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};
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struct kvm_shared_msrs {
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struct user_return_notifier urn;
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bool registered;
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u64 current_value[KVM_NR_SHARED_MSRS];
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};
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static struct kvm_shared_msrs_global __read_mostly shared_msrs_global;
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static DEFINE_PER_CPU(struct kvm_shared_msrs, shared_msrs);
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struct kvm_stats_debugfs_item debugfs_entries[] = {
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{ "pf_fixed", VCPU_STAT(pf_fixed) },
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{ "pf_guest", VCPU_STAT(pf_guest) },
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@ -123,6 +143,64 @@ struct kvm_stats_debugfs_item debugfs_entries[] = {
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{ NULL }
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};
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static void kvm_on_user_return(struct user_return_notifier *urn)
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{
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unsigned slot;
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struct kvm_shared_msr *global;
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struct kvm_shared_msrs *locals
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= container_of(urn, struct kvm_shared_msrs, urn);
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for (slot = 0; slot < shared_msrs_global.nr; ++slot) {
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global = &shared_msrs_global.msrs[slot];
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if (global->value != locals->current_value[slot]) {
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wrmsrl(global->msr, global->value);
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locals->current_value[slot] = global->value;
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}
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}
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locals->registered = false;
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user_return_notifier_unregister(urn);
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}
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void kvm_define_shared_msr(unsigned slot, u32 msr)
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{
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int cpu;
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u64 value;
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if (slot >= shared_msrs_global.nr)
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shared_msrs_global.nr = slot + 1;
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shared_msrs_global.msrs[slot].msr = msr;
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rdmsrl_safe(msr, &value);
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shared_msrs_global.msrs[slot].value = value;
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for_each_online_cpu(cpu)
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per_cpu(shared_msrs, cpu).current_value[slot] = value;
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}
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EXPORT_SYMBOL_GPL(kvm_define_shared_msr);
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static void kvm_shared_msr_cpu_online(void)
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{
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unsigned i;
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struct kvm_shared_msrs *locals = &__get_cpu_var(shared_msrs);
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for (i = 0; i < shared_msrs_global.nr; ++i)
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locals->current_value[i] = shared_msrs_global.msrs[i].value;
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}
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void kvm_set_shared_msr(unsigned slot, u64 value)
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{
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struct kvm_shared_msrs *smsr = &__get_cpu_var(shared_msrs);
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if (value == smsr->current_value[slot])
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return;
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smsr->current_value[slot] = value;
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wrmsrl(shared_msrs_global.msrs[slot].msr, value);
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if (!smsr->registered) {
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smsr->urn.on_user_return = kvm_on_user_return;
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user_return_notifier_register(&smsr->urn);
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smsr->registered = true;
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}
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}
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EXPORT_SYMBOL_GPL(kvm_set_shared_msr);
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unsigned long segment_base(u16 selector)
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{
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struct descriptor_table gdt;
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@ -4815,6 +4893,9 @@ int kvm_arch_hardware_enable(void *garbage)
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int cpu = raw_smp_processor_id();
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per_cpu(cpu_tsc_khz, cpu) = 0;
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}
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kvm_shared_msr_cpu_online();
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return kvm_x86_ops->hardware_enable(garbage);
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}
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