2020-03-13 03:57:05

by Wanpeng Li

Subject: [PATCH v2] KVM: VMX: Micro-optimize vmexit time when not exposing PMU

From: Wanpeng Li <[email protected]>

PMU is not exposed to guest by most of products from cloud providers since the
bad performance of PMU emulation and security concern. However, it calls
perf_guest_switch_get_msrs() and clear_atomic_switch_msr() unconditionally
even if PMU is not exposed to the guest before each vmentry.

~2% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my
SKX server.

Before patch:
vmcall 1559

After patch:
vmcall 1529

Signed-off-by: Wanpeng Li <[email protected]>
---
v1 -> v2:
* move the check before atomic_switch_perf_msrs

arch/x86/kvm/vmx/vmx.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
index 40b1e61..b20423c 100644
--- a/arch/x86/kvm/vmx/vmx.c
+++ b/arch/x86/kvm/vmx/vmx.c
@@ -6567,7 +6567,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)

pt_guest_enter(vmx);

- atomic_switch_perf_msrs(vmx);
+ if (vcpu_to_pmu(vcpu)->version)
+ atomic_switch_perf_msrs(vmx);
atomic_switch_umwait_control_msr(vmx);

if (enable_preemption_timer)
--
2.7.4

2020-03-15 03:03:38

by Paolo Bonzini

Subject: Re: [PATCH v2] KVM: VMX: Micro-optimize vmexit time when not exposing PMU

On 13/03/20 04:55, Wanpeng Li wrote:
> From: Wanpeng Li <[email protected]>
>
> PMU is not exposed to guest by most of products from cloud providers since the
> bad performance of PMU emulation and security concern. However, it calls
> perf_guest_switch_get_msrs() and clear_atomic_switch_msr() unconditionally
> even if PMU is not exposed to the guest before each vmentry.
>
> ~2% vmexit time reduced can be observed by kvm-unit-tests/vmexit.flat on my
> SKX server.
>
> Before patch:
> vmcall 1559
>
> After patch:
> vmcall 1529
>
> Signed-off-by: Wanpeng Li <[email protected]>
> ---
> v1 -> v2:
> * move the check before atomic_switch_perf_msrs
>
> arch/x86/kvm/vmx/vmx.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 40b1e61..b20423c 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -6567,7 +6567,8 @@ static void vmx_vcpu_run(struct kvm_vcpu *vcpu)
>
> pt_guest_enter(vmx);
>
> - atomic_switch_perf_msrs(vmx);
> + if (vcpu_to_pmu(vcpu)->version)
> + atomic_switch_perf_msrs(vmx);
> atomic_switch_umwait_control_msr(vmx);
>
> if (enable_preemption_timer)
>

Queued, thanks.

Paolo