Re: [PATCH v3 08/12] KVM: X86: Move write_l1_tsc_offset() logic to common code and rename it

[Maxim Levitsky]

On Fri, 2021-05-21 at 11:24 +0100, Ilias Stamatis wrote:
> The write_l1_tsc_offset() callback has a misleading name. It does not
> set L1's TSC offset, it rather updates the current TSC offset which
> might be different if a nested guest is executing. Additionally, both
> the vmx and svm implementations use the same logic for calculating the
> current TSC before writing it to hardware.
> 
> Rename the function and move the common logic to the caller. The vmx/svm
> specific code now merely sets the given offset to the corresponding
> hardware structure.
> 
> Signed-off-by: Ilias Stamatis <ilstam@xxxxxxxxxx>
> ---
>  arch/x86/include/asm/kvm-x86-ops.h |  2 +-
>  arch/x86/include/asm/kvm_host.h    |  3 +--
>  arch/x86/kvm/svm/svm.c             | 21 ++++-----------------
>  arch/x86/kvm/vmx/vmx.c             | 23 +++--------------------
>  arch/x86/kvm/x86.c                 | 24 +++++++++++++++++++++---
>  5 files changed, 30 insertions(+), 43 deletions(-)
> 
> diff --git a/arch/x86/include/asm/kvm-x86-ops.h b/arch/x86/include/asm/kvm-x86-ops.h
> index 2063616fba1c..029c9615378f 100644
> --- a/arch/x86/include/asm/kvm-x86-ops.h
> +++ b/arch/x86/include/asm/kvm-x86-ops.h
> @@ -89,7 +89,7 @@ KVM_X86_OP(load_mmu_pgd)
>  KVM_X86_OP_NULL(has_wbinvd_exit)
>  KVM_X86_OP(get_l2_tsc_offset)
>  KVM_X86_OP(get_l2_tsc_multiplier)
> -KVM_X86_OP(write_l1_tsc_offset)
> +KVM_X86_OP(write_tsc_offset)
>  KVM_X86_OP(get_exit_info)
>  KVM_X86_OP(check_intercept)
>  KVM_X86_OP(handle_exit_irqoff)
> diff --git a/arch/x86/include/asm/kvm_host.h b/arch/x86/include/asm/kvm_host.h
> index aaf756442ed1..f099277b993d 100644
> --- a/arch/x86/include/asm/kvm_host.h
> +++ b/arch/x86/include/asm/kvm_host.h
> @@ -1307,8 +1307,7 @@ struct kvm_x86_ops {
>  
>  	u64 (*get_l2_tsc_offset)(struct kvm_vcpu *vcpu);
>  	u64 (*get_l2_tsc_multiplier)(struct kvm_vcpu *vcpu);
> -	/* Returns actual tsc_offset set in active VMCS */
> -	u64 (*write_l1_tsc_offset)(struct kvm_vcpu *vcpu, u64 offset);
> +	void (*write_tsc_offset)(struct kvm_vcpu *vcpu, u64 offset);
>  
>  	/*
>  	 * Retrieve somewhat arbitrary exit information.  Intended to be used
> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> index ca70e46f9194..8dfb2513b72a 100644
> --- a/arch/x86/kvm/svm/svm.c
> +++ b/arch/x86/kvm/svm/svm.c
> @@ -1094,26 +1094,13 @@ static u64 svm_get_l2_tsc_multiplier(struct kvm_vcpu *vcpu)
>  	return kvm_default_tsc_scaling_ratio;
>  }
>  
> -static u64 svm_write_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
> +static void svm_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
>  {
>  	struct vcpu_svm *svm = to_svm(vcpu);
> -	u64 g_tsc_offset = 0;
> -
> -	if (is_guest_mode(vcpu)) {
> -		/* Write L1's TSC offset.  */
> -		g_tsc_offset = svm->vmcb->control.tsc_offset -
> -			       svm->vmcb01.ptr->control.tsc_offset;
> -		svm->vmcb01.ptr->control.tsc_offset = offset;
> -	}
> -
> -	trace_kvm_write_tsc_offset(vcpu->vcpu_id,
> -				   svm->vmcb->control.tsc_offset - g_tsc_offset,
> -				   offset);
> -
> -	svm->vmcb->control.tsc_offset = offset + g_tsc_offset;
>  
> +	svm->vmcb01.ptr->control.tsc_offset = vcpu->arch.l1_tsc_offset;
> +	svm->vmcb->control.tsc_offset = offset;
>  	vmcb_mark_dirty(svm->vmcb, VMCB_INTERCEPTS);
> -	return svm->vmcb->control.tsc_offset;
>  }
>  
>  /* Evaluate instruction intercepts that depend on guest CPUID features. */
> @@ -4540,7 +4527,7 @@ static struct kvm_x86_ops svm_x86_ops __initdata = {
>  
>  	.get_l2_tsc_offset = svm_get_l2_tsc_offset,
>  	.get_l2_tsc_multiplier = svm_get_l2_tsc_multiplier,
> -	.write_l1_tsc_offset = svm_write_l1_tsc_offset,
> +	.write_tsc_offset = svm_write_tsc_offset,
>  
>  	.load_mmu_pgd = svm_load_mmu_pgd,
>  
> diff --git a/arch/x86/kvm/vmx/vmx.c b/arch/x86/kvm/vmx/vmx.c
> index 1c83605eccc1..4b70431c2edd 100644
> --- a/arch/x86/kvm/vmx/vmx.c
> +++ b/arch/x86/kvm/vmx/vmx.c
> @@ -1808,26 +1808,9 @@ u64 vmx_get_l2_tsc_multiplier(struct kvm_vcpu *vcpu)
>  	return kvm_default_tsc_scaling_ratio;
>  }
>  
> -static u64 vmx_write_l1_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
> +static void vmx_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
>  {
> -	struct vmcs12 *vmcs12 = get_vmcs12(vcpu);
> -	u64 g_tsc_offset = 0;
> -
> -	/*
> -	 * We're here if L1 chose not to trap WRMSR to TSC. According
> -	 * to the spec, this should set L1's TSC; The offset that L1
> -	 * set for L2 remains unchanged, and still needs to be added
> -	 * to the newly set TSC to get L2's TSC.
> -	 */
> -	if (is_guest_mode(vcpu) &&
> -	    (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETTING))
> -		g_tsc_offset = vmcs12->tsc_offset;
> -
> -	trace_kvm_write_tsc_offset(vcpu->vcpu_id,
> -				   vcpu->arch.tsc_offset - g_tsc_offset,
> -				   offset);
> -	vmcs_write64(TSC_OFFSET, offset + g_tsc_offset);
> -	return offset + g_tsc_offset;
> +	vmcs_write64(TSC_OFFSET, offset);
>  }
>  
>  /*
> @@ -7723,7 +7706,7 @@ static struct kvm_x86_ops vmx_x86_ops __initdata = {
>  
>  	.get_l2_tsc_offset = vmx_get_l2_tsc_offset,
>  	.get_l2_tsc_multiplier = vmx_get_l2_tsc_multiplier,
> -	.write_l1_tsc_offset = vmx_write_l1_tsc_offset,
> +	.write_tsc_offset = vmx_write_tsc_offset,
>  
>  	.load_mmu_pgd = vmx_load_mmu_pgd,
>  
> diff --git a/arch/x86/kvm/x86.c b/arch/x86/kvm/x86.c
> index 04abaacb9cfc..2f91259070e9 100644
> --- a/arch/x86/kvm/x86.c
> +++ b/arch/x86/kvm/x86.c
> @@ -2359,10 +2359,28 @@ u64 kvm_calc_nested_tsc_multiplier(u64 l1_multiplier, u64 l2_multiplier)
>  }
>  EXPORT_SYMBOL_GPL(kvm_calc_nested_tsc_multiplier);
>  
> -static void kvm_vcpu_write_tsc_offset(struct kvm_vcpu *vcpu, u64 offset)
> +static void kvm_vcpu_write_tsc_offset(struct kvm_vcpu *vcpu, u64 l1_offset)
>  {
> -	vcpu->arch.l1_tsc_offset = offset;
> -	vcpu->arch.tsc_offset = static_call(kvm_x86_write_l1_tsc_offset)(vcpu, offset);
> +	trace_kvm_write_tsc_offset(vcpu->vcpu_id,
> +				   vcpu->arch.l1_tsc_offset,
> +				   l1_offset);
> +
> +	vcpu->arch.l1_tsc_offset = l1_offset;
> +
> +	/*
> +	 * If we are here because L1 chose not to trap WRMSR to TSC then
> +	 * according to the spec this should set L1's TSC (as opposed to
> +	 * setting L1's offset for L2).
> +	 */
> +	if (is_guest_mode(vcpu))
> +		vcpu->arch.tsc_offset = kvm_calc_nested_tsc_offset(
> +			l1_offset,
> +			static_call(kvm_x86_get_l2_tsc_offset)(vcpu),
> +			static_call(kvm_x86_get_l2_tsc_multiplier)(vcpu));
> +	else
> +		vcpu->arch.tsc_offset = l1_offset;
> +
> +	static_call(kvm_x86_write_tsc_offset)(vcpu, vcpu->arch.tsc_offset);
>  }
>  
>  static inline bool kvm_check_tsc_unstable(void)


Reviewed-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>

Best regards,
	Maxim Levitsky