[PATCH] KVM: VMX: Cache IA32_DEBUGCTL in memory

[Wanpeng Li]

From: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>

MSR_IA32_DEBUGCTLMSR is zeroed on VMEXIT, so it is saved/restored 
each time during world switch. Jim from Google pointed out that 
when running schbench in L2, vmx_vcpu_run will occupy 4% cpu time, 
and the 25% of vmx_vcpu_run cpu time is occupied by get_debugctlmsr(). 
This patch caches the host IA32_DEBUGCTL MSR and saves/restores 
the host IA32_DEBUGCTL msr when guest/host switches to avoid to 
save/restore each time during world switch.

Suggested-by: Jim Mattson <jmattson@xxxxxxxxxx>
Cc: Jim Mattson <jmattson@xxxxxxxxxx>
Cc: Paolo Bonzini <pbonzini@xxxxxxxxxx>
Cc: Radim KrÄmÃÅ <rkrcmar@xxxxxxxxxx>
Signed-off-by: Wanpeng Li <wanpeng.li@xxxxxxxxxxx>
---
 arch/x86/include/asm/kvm_host.h |  1 +
 arch/x86/kvm/vmx.c              | 11 +++++------
 2 files changed, 6 insertions(+), 6 deletions(-)

diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
index 63d34bc..c904250 100644
--- a/arch/x86/include/asm/kvm_host.h
+++ b/arch/x86/include/asm/kvm_host.h
@@ -619,6 +619,7 @@ struct kvm_vcpu_arch {
 	unsigned long dr7;
 	unsigned long eff_db[KVM_NR_DB_REGS];
 	unsigned long guest_debug_dr7;
+	unsigned long debugctlmsr;
 	u64 msr_platform_info;
 	u64 msr_misc_features_enables;
 
diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 8c7e816..b167bba 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -2326,6 +2326,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)
 
 	vmx_vcpu_pi_load(vcpu, cpu);
 	vmx->host_pkru = read_pkru();
+	vcpu->arch.debugctlmsr = get_debugctlmsr();
 }
 
 static void vmx_vcpu_pi_put(struct kvm_vcpu *vcpu)
@@ -2347,6 +2348,9 @@ static void vmx_vcpu_put(struct kvm_vcpu *vcpu)
 	vmx_vcpu_pi_put(vcpu);
 
 	__vmx_load_host_state(to_vmx(vcpu));
+	/* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
+	if (vcpu->arch.debugctlmsr)
+		update_debugctlmsr(vcpu->arch.debugctlmsr);
 }
 
 static bool emulation_required(struct kvm_vcpu *vcpu)
@@ -9346,7 +9350,7 @@ static void vmx_arm_hv_timer(struct kvm_vcpu *vcpu)
 static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 {
 	struct vcpu_vmx *vmx = to_vmx(vcpu);
-	unsigned long debugctlmsr, cr3, cr4;
+	unsigned long cr3, cr4;
 
 	/* Record the guest's net vcpu time for enforced NMI injections. */
 	if (unlikely(!enable_vnmi &&
@@ -9399,7 +9403,6 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 		__write_pkru(vcpu->arch.pkru);
 
 	atomic_switch_perf_msrs(vmx);
-	debugctlmsr = get_debugctlmsr();
 
 	vmx_arm_hv_timer(vcpu);
 
@@ -9509,10 +9512,6 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
 #endif
 	      );
 
-	/* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
-	if (debugctlmsr)
-		update_debugctlmsr(debugctlmsr);
-
 #ifndef CONFIG_X86_64
 	/*
 	 * The sysexit path does not restore ds/es, so we must set them to
-- 
2.7.4