[PATCH][SVM] Lazy fast syscall MSR save/restore

[Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>]

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Howdy,

The attached patch only reloads the host fast syscall MSRs when we leave 
KVM (when we put_cpu()).  Since we'll never execute a 
sys{enter,call,ret,exit} until we put_cpu() in the host, this should be 
safe.

This has a huge impact on in-kernel vmexit performance.  I see a savings 
of about a 1,000 cycles going from 4,700 to about 3,700 or ~21%.

I've only tested on a 64bit host with a 32bit FC6 guest but I wanted to 
get it out before calling it a night.  I had fumbled my previous attempt 
at this by switching the rdmsr/wrmsrs around so ignore my previous 
rambles about improperly blocking :-)

Regards,

Anthony Liguori

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From: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>
Subject: [PATCH][SVM] Only save fast syscall MSRS when leaving VCPU

  We only need to restore the host fast syscall related MSRS when leaving the
  VCPU.  This should only occur when we {get,put}_cpu().

Signed-off-by: Anthony Liguori <aliguori-r/Jw6+rmf7HQT0dZR+AlfA@public.gmane.org>

Index: kernel/drivers/kvm/svm.c
===================================================================
--- kernel.orig/drivers/kvm/svm.c	2007-04-26 18:07:09.909905480 -0500
+++ kernel/drivers/kvm/svm.c	2007-04-27 00:38:57.671182560 -0500
@@ -611,7 +611,7 @@
 
 static void svm_vcpu_load(struct kvm_vcpu *vcpu)
 {
-	int cpu;
+	int cpu, i;
 
 	cpu = get_cpu();
 	if (unlikely(cpu != vcpu->cpu)) {
@@ -626,10 +626,18 @@
 		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();
 }
Index: kernel/drivers/kvm/kvm_svm.h
===================================================================
--- kernel.orig/drivers/kvm/kvm_svm.h	2007-04-26 18:07:09.949899400 -0500
+++ kernel/drivers/kvm/kvm_svm.h	2007-04-27 00:38:30.310342040 -0500
@@ -11,15 +11,19 @@
 
 static const u32 host_save_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_KERNEL_GS_BASE, MSR_FS_BASE, MSR_GS_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,*/
 };
 
+static const u32 host_save_user_msrs[] = {
+	MSR_STAR, MSR_LSTAR, MSR_CSTAR, MSR_SYSCALL_MASK,
+	MSR_IA32_SYSENTER_CS, MSR_IA32_SYSENTER_ESP, MSR_IA32_SYSENTER_EIP,
+};
+
 #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 {
@@ -33,6 +37,7 @@
 	u64 next_rip;
 
 	u64 host_msrs[NR_HOST_SAVE_MSRS];
+	u64 host_user_msrs[NR_HOST_SAVE_USER_MSRS];
 	unsigned long host_cr2;
 	unsigned long host_db_regs[NUM_DB_REGS];
 	unsigned long host_dr6;

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