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>

1 files changed, 5 insertions, 6 deletions

diff --git a/drivers/kvm/kvm_svm.h b/drivers/kvm/kvm_svm.h
index a1a9eba50d47..a869983d683d 100644
--- a/drivers/kvm/kvm_svm.h
+++ b/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;