linux-kernel - Re: [PATCH v3 09/12] KVM: VMX: Remove vmx->current_tsc_ratio and decache_tsc

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Date:   Mon, 24 May 2021 20:53:54 +0300
From:   Maxim Levitsky <mlevitsk@...hat.com>
To:     Ilias Stamatis <ilstam@...zon.com>, kvm@...r.kernel.org,
        linux-kernel@...r.kernel.org, pbonzini@...hat.com
Cc:     seanjc@...gle.com, vkuznets@...hat.com, wanpengli@...cent.com,
        jmattson@...gle.com, joro@...tes.org, zamsden@...il.com,
        mtosatti@...hat.com, dwmw@...zon.co.uk
Subject: Re: [PATCH v3 09/12] KVM: VMX: Remove vmx->current_tsc_ratio and
 decache_tsc_multiplier()

On Fri, 2021-05-21 at 11:24 +0100, Ilias Stamatis wrote:
> The vmx->current_tsc_ratio field is redundant as
> vcpu->arch.tsc_scaling_ratio already tracks the current TSC scaling
> ratio. Removing this field makes decache_tsc_multiplier() an one-liner
> so remove that too and do a vmcs_write64() directly in order to be more
> consistent with surrounding code.
Not to mention that 'decache_tsc_multiplier' isn't a good name IMHO
for this....


> 
> Signed-off-by: Ilias Stamatis <ilstam@...zon.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 9 ++++-----
>  arch/x86/kvm/vmx/vmx.c    | 5 ++---
>  arch/x86/kvm/vmx/vmx.h    | 8 --------
>  3 files changed, 6 insertions(+), 16 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 6058a65a6ede..239154d3e4e7 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -2533,9 +2533,8 @@ static int prepare_vmcs02(struct kvm_vcpu *vcpu, struct vmcs12 *vmcs12,
>  	}
>  
>  	vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> -
>  	if (kvm_has_tsc_control)
> -		decache_tsc_multiplier(vmx);
> +		vmcs_write64(TSC_MULTIPLIER, vcpu->arch.tsc_scaling_ratio);
>  
>  	nested_vmx_transition_tlb_flush(vcpu, vmcs12, true);
>  
> @@ -4501,12 +4500,12 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
>  	vmcs_write32(VM_EXIT_MSR_LOAD_COUNT, vmx->msr_autoload.host.nr);
>  	vmcs_write32(VM_ENTRY_MSR_LOAD_COUNT, vmx->msr_autoload.guest.nr);
>  	vmcs_write64(TSC_OFFSET, vcpu->arch.tsc_offset);
> +	if (kvm_has_tsc_control)
> +		vmcs_write64(TSC_MULTIPLIER, vcpu->arch.tsc_scaling_ratio);
> +
>  	if (vmx->nested.l1_tpr_threshold != -1)
>  		vmcs_write32(TPR_THRESHOLD, vmx->nested.l1_tpr_threshold);
>  
> -	if (kvm_has_tsc_control)
> -		decache_tsc_multiplier(vmx);
> -
>  	if (vmx->nested.change_vmcs01_virtual_apic_mode) {
>  		vmx->nested.change_vmcs01_virtual_apic_mode = false;
>  		vmx_set_virtual_apic_mode(vcpu);
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 4b70431c2edd..7c52c697cfe3 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1392,9 +1392,8 @@ void vmx_vcpu_load_vmcs(struct kvm_vcpu *vcpu, int cpu,
>  	}
>  
>  	/* Setup TSC multiplier */
> -	if (kvm_has_tsc_control &&
> -	    vmx->current_tsc_ratio != vcpu->arch.tsc_scaling_ratio)
> -		decache_tsc_multiplier(vmx);
> +	if (kvm_has_tsc_control)
> +		vmcs_write64(TSC_MULTIPLIER, vcpu->arch.tsc_scaling_ratio);

This might have an overhead of writing the TSC scaling ratio even if
it is unchanged. I haven't measured how expensive vmread/vmwrites are but
at least when nested, the vmreads/vmwrites can be very expensive (if they
cause a vmexit).

This is why I think the 'vmx->current_tsc_ratio' exists - to have
a cached value of TSC scale ratio to avoid either 'vmread'ing
or 'vmwrite'ing it without a need.


Best regards,
	Maxim Levitsky

>  }
>  
>  /*
> diff --git a/arch/x86/kvm/vmx/vmx.h b/arch/x86/kvm/vmx/vmx.h
> index aa97c82e3451..3eaa86a0ba3e 100644
> --- a/arch/x86/kvm/vmx/vmx.h
> +++ b/arch/x86/kvm/vmx/vmx.h
> @@ -322,8 +322,6 @@ struct vcpu_vmx {
>  	/* apic deadline value in host tsc */
>  	u64 hv_deadline_tsc;
>  
> -	u64 current_tsc_ratio;
> -
>  	unsigned long host_debugctlmsr;
>  
>  	/*
> @@ -532,12 +530,6 @@ static inline struct vmcs *alloc_vmcs(bool shadow)
>  			      GFP_KERNEL_ACCOUNT);
>  }
>  
> -static inline void decache_tsc_multiplier(struct vcpu_vmx *vmx)
> -{
> -	vmx->current_tsc_ratio = vmx->vcpu.arch.tsc_scaling_ratio;
> -	vmcs_write64(TSC_MULTIPLIER, vmx->current_tsc_ratio);
> -}
> -
>  static inline bool vmx_has_waitpkg(struct vcpu_vmx *vmx)
>  {
>  	return vmx->secondary_exec_control &