KVM: VMX: Reduce unnecessary saving of host msrs
THe automatically switched msrs are never changed on the host (with
the exception of MSR_KERNEL_GS_BASE) and thus there is no need to save
them on every vm entry.
This reduces vmexit latency by ~400 cycles on i386 and by ~900 cycles (10%)
on x86_64.
Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/drivers/kvm/vmx.c b/drivers/kvm/vmx.c
index 8c0115b..3745e6c 100644
--- a/drivers/kvm/vmx.c
+++ b/drivers/kvm/vmx.c
@@ -78,6 +78,10 @@
};
#define NR_VMX_MSR ARRAY_SIZE(vmx_msr_index)
+#ifdef CONFIG_X86_64
+static unsigned msr_offset_kernel_gs_base;
+#endif
+
static inline int is_page_fault(u32 intr_info)
{
return (intr_info & (INTR_INFO_INTR_TYPE_MASK | INTR_INFO_VECTOR_MASK |
@@ -1129,6 +1133,10 @@
vcpu->host_msrs[j].reserved = 0;
vcpu->host_msrs[j].data = data;
vcpu->guest_msrs[j] = vcpu->host_msrs[j];
+#ifdef CONFIG_X86_64
+ if (index == MSR_KERNEL_GS_BASE)
+ msr_offset_kernel_gs_base = j;
+#endif
++vcpu->nmsrs;
}
@@ -1760,7 +1768,9 @@
fx_save(vcpu->host_fx_image);
fx_restore(vcpu->guest_fx_image);
- save_msrs(vcpu->host_msrs, vcpu->nmsrs);
+#ifdef CONFIG_X86_64
+ save_msrs(vcpu->host_msrs + msr_offset_kernel_gs_base, 1);
+#endif
load_msrs(vcpu->guest_msrs, NR_BAD_MSRS);
asm (
