linux-kernel - Re: [PATCH v2] KVM: VMX: Cache IA32

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Date:   Wed, 29 Nov 2017 18:32:18 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Paolo Bonzini <pbonzini@...hat.com>,
        Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH v2] KVM: VMX: Cache IA32_DEBUGCTL in memory

On 29.11.2017 10:31, Wanpeng Li wrote:
> From: Wanpeng Li <wanpeng.li@...mail.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@...gle.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Cc: Jim Mattson <jmattson@...gle.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.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@...hat.com>

-- 

Thanks,

David / dhildenb