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

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Date:   Wed, 29 Nov 2017 09:48:43 +0100
From:   Paolo Bonzini <pbonzini@...hat.com>
To:     Wanpeng Li <kernellwp@...il.com>, linux-kernel@...r.kernel.org,
        kvm@...r.kernel.org
Cc:     Radim Krčmář <rkrcmar@...hat.com>,
        Wanpeng Li <wanpeng.li@...mail.com>,
        Jim Mattson <jmattson@...gle.com>
Subject: Re: [PATCH] KVM: VMX: Cache IA32_DEBUGCTL in memory

On 29/11/2017 07:07, 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: Jim Mattson <jmattson@...gle.com>
> Cc: Paolo Bonzini <pbonzini@...hat.com>
> Cc: Radim Krčmář <rkrcmar@...hat.com>
> Signed-off-by: Wanpeng Li <wanpeng.li@...mail.com>

The update_debugctlmsr should stay in vmx_vcpu_run so that tracing
features work correctly.  However, the get_debugctlmsr indeed can be
moved to vmx_vcpu_load.

> ---
>  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;

Please rename to host_debugctlmsr and place it in struct vcpu_vmx.

Thanks,

Paolo

>  	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
>