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

David Hildenbrand
2017-11-29T17:32:18Z

On 29.11.2017 10:31, Wanpeng Li wrote:
> 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
> /*
>

Reviewed-by: David Hildenbrand <david@redhat.com>

--

Thanks,

David / dhildenb