[PATCH v2] KVM: VMX: Cache IA32_DEBUGCTL in memory

Wanpeng Li
2017-11-29T09:31:20Z

From: Wanpeng Li <wanpeng.li@hotmail.com>

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@google.com>
Cc: Paolo Bonzini <pbonzini@redhat.com>
Cc: Radim Krčmář <rkrcmar@redhat.com>
Cc: Jim Mattson <jmattson@google.com>
Signed-off-by: Wanpeng Li <wanpeng.li@hotmail.com>
---
v1 -> v2:
* rename to host_debugctlmsr and place it in struct vcpu_struct
* update_debugctlmsr stay in vmx_vcpu_run

arch/x86/kvm/vmx.c | 10 ++++++----
1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/arch/x86/kvm/vmx.c b/arch/x86/kvm/vmx.c
index 8c7e816..d293c29 100644
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -658,6 +658,8 @@ struct vcpu_vmx {

u32 host_pkru;

+ unsigned long host_debugctlmsr;
+
/*
* Only bits masked by msr_ia32_feature_control_valid_bits can be set in
* msr_ia32_feature_control. FEATURE_CONTROL_LOCKED is always included
@@ -2326,6 +2328,7 @@ static void vmx_vcpu_load(struct kvm_vcpu *vcpu, int cpu)

vmx_vcpu_pi_load(vcpu, cpu);
vmx->host_pkru = read_pkru();
+ vmx->host_debugctlmsr = get_debugctlmsr();
}

static void vmx_vcpu_pi_put(struct kvm_vcpu *vcpu)
@@ -9346,7 +9349,7 @@ 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 +9402,6 @@ 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);

@@ -9510,8 +9512,8 @@ static void __noclone vmx_vcpu_run(struct kvm_vcpu *vcpu)
);

/* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
- if (debugctlmsr)
- update_debugctlmsr(debugctlmsr);
+ if (vmx->host_debugctlmsr)
+ update_debugctlmsr(vmx->host_debugctlmsr);

#ifndef CONFIG_X86_64
/*
--
2.7.4