KVM: VMX: Only save/restore MSR_K6_STAR if necessary
Intel hosts only support syscall/sysret in long more (and only if efer.sce
is enabled), so only reload the related MSR_K6_STAR if the guest will
actually be able to use it.
This reduces vmexit cost by about 500 cycles (6400 -> 5870) on my setup.
Signed-off-by: Avi Kivity <avi@qumranet.com>
diff --git a/drivers/kvm/vmx.c b/drivers/kvm/vmx.c
index b61d4dd..37537af 100644
--- a/drivers/kvm/vmx.c
+++ b/drivers/kvm/vmx.c
@@ -69,6 +69,10 @@
VMX_SEGMENT_FIELD(LDTR),
};
+/*
+ * Keep MSR_K6_STAR at the end, as setup_msrs() will try to optimize it
+ * away by decrementing the array size.
+ */
static const u32 vmx_msr_index[] = {
#ifdef CONFIG_X86_64
MSR_SYSCALL_MASK, MSR_LSTAR, MSR_CSTAR, MSR_KERNEL_GS_BASE,
@@ -323,6 +327,18 @@
nr_skip = NR_64BIT_MSRS;
nr_good_msrs = vcpu->nmsrs - nr_skip;
+ /*
+ * MSR_K6_STAR is only needed on long mode guests, and only
+ * if efer.sce is enabled.
+ */
+ if (find_msr_entry(vcpu, MSR_K6_STAR)) {
+ --nr_good_msrs;
+#ifdef CONFIG_X86_64
+ if (is_long_mode(vcpu) && (vcpu->shadow_efer & EFER_SCE))
+ ++nr_good_msrs;
+#endif
+ }
+
vmcs_writel(VM_ENTRY_MSR_LOAD_ADDR,
virt_to_phys(vcpu->guest_msrs + nr_skip));
vmcs_writel(VM_EXIT_MSR_STORE_ADDR,
