KVM: SVM: Only save/restore MSRs when needed

We only have to save/restore MSR_GS_BASE on every VMEXIT.  The rest can be
saved/restored when we leave the VCPU.  Since we don't emulate the DEBUGCTL
MSRs and the guest cannot write to them, we don't have to worry about
saving/restoring them at all.

This shaves a whopping 40% off raw vmexit costs on AMD.
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>
Signed-off-by: Avi Kivity <avi@qumranet.com>

drivers/kvm/kvm_svm.h

@@ -9,17 +9,15 @@
 #include "svm.h"
 #include "kvm.h"
 
-static const u32 host_save_msrs[] = {
+static const u32 host_save_user_msrs[] = {
 #ifdef CONFIG_X86_64
 	MSR_STAR, MSR_LSTAR, MSR_CSTAR, MSR_SYSCALL_MASK, MSR_KERNEL_GS_BASE,
-	MSR_FS_BASE, MSR_GS_BASE,
+	MSR_FS_BASE,
 #endif
 	MSR_IA32_SYSENTER_CS, MSR_IA32_SYSENTER_ESP, MSR_IA32_SYSENTER_EIP,
-	MSR_IA32_DEBUGCTLMSR, /*MSR_IA32_LASTBRANCHFROMIP,
-	MSR_IA32_LASTBRANCHTOIP, MSR_IA32_LASTINTFROMIP,MSR_IA32_LASTINTTOIP,*/
 };
 
-#define NR_HOST_SAVE_MSRS ARRAY_SIZE(host_save_msrs)
+#define NR_HOST_SAVE_USER_MSRS ARRAY_SIZE(host_save_user_msrs)
 #define NUM_DB_REGS 4
 
 struct vcpu_svm {
@@ -32,7 +30,8 @@ struct vcpu_svm {
 
 	u64 next_rip;
 
-	u64 host_msrs[NR_HOST_SAVE_MSRS];
+	u64 host_user_msrs[NR_HOST_SAVE_USER_MSRS];
+	u64 host_gs_base;
 	unsigned long host_cr2;
 	unsigned long host_db_regs[NUM_DB_REGS];
 	unsigned long host_dr6;

drivers/kvm/svm.c

@@ -522,8 +522,6 @@ static void init_vmcb(struct vmcb *vmcb)
 	control->msrpm_base_pa = msrpm_base;
 	control->tsc_offset = 0;
 	control->int_ctl = V_INTR_MASKING_MASK;
-	if (svm_has(SVM_FEATURE_LBRV))
-		control->lbr_ctl = 1ULL;
 
 	init_seg(&save->es);
 	init_seg(&save->ss);
@@ -611,7 +609,7 @@ static void svm_free_vcpu(struct kvm_vcpu *vcpu)
 
 static void svm_vcpu_load(struct kvm_vcpu *vcpu)
 {
-	int cpu;
+	int cpu, i;
 
 	cpu = get_cpu();
 	if (unlikely(cpu != vcpu->cpu)) {
@@ -626,10 +624,18 @@ static void svm_vcpu_load(struct kvm_vcpu *vcpu)
 		vcpu->svm->vmcb->control.tsc_offset += delta;
 		vcpu->cpu = cpu;
 	}
+
+	for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
+		rdmsrl(host_save_user_msrs[i], vcpu->svm->host_user_msrs[i]);
 }
 
 static void svm_vcpu_put(struct kvm_vcpu *vcpu)
 {
+	int i;
+
+	for (i = 0; i < NR_HOST_SAVE_USER_MSRS; i++)
+		wrmsrl(host_save_user_msrs[i], vcpu->svm->host_user_msrs[i]);
+
 	rdtscll(vcpu->host_tsc);
 	put_cpu();
 }
@@ -815,18 +821,16 @@ static int svm_guest_debug(struct kvm_vcpu *vcpu, struct kvm_debug_guest *dbg)
 
 static void load_host_msrs(struct kvm_vcpu *vcpu)
 {
-	int i;
-
-	for ( i = 0; i < NR_HOST_SAVE_MSRS; i++)
-		wrmsrl(host_save_msrs[i], vcpu->svm->host_msrs[i]);
+#ifdef CONFIG_X86_64
+	wrmsrl(MSR_GS_BASE, vcpu->svm->host_gs_base);
+#endif
 }
 
 static void save_host_msrs(struct kvm_vcpu *vcpu)
 {
-	int i;
-
-	for ( i = 0; i < NR_HOST_SAVE_MSRS; i++)
-		rdmsrl(host_save_msrs[i], vcpu->svm->host_msrs[i]);
+#ifdef CONFIG_X86_64
+	rdmsrl(MSR_GS_BASE, vcpu->svm->host_gs_base);
+#endif
 }
 
 static void new_asid(struct kvm_vcpu *vcpu, struct svm_cpu_data *svm_data)