linux-kernel - Re: [PATCH v2 08/10] KVM: VMX: Set the TSC offset and multiplier on nested entry and exit

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Date:   Wed, 19 May 2021 00:07:56 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Ilias Stamatis <ilstam@...zon.com>
Cc:     kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
        pbonzini@...hat.com, mlevitsk@...hat.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 v2 08/10] KVM: VMX: Set the TSC offset and multiplier on
 nested entry and exit

On Wed, May 12, 2021, Ilias Stamatis wrote:
> Now that nested TSC scaling is supported we need to calculate the
> correct 02 values for both the offset and the multiplier using the
> corresponding functions. On L2's exit the L1 values are restored.
> 
> Signed-off-by: Ilias Stamatis <ilstam@...zon.com>
> ---
>  arch/x86/kvm/vmx/nested.c | 13 +++++++++----
>  1 file changed, 9 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/vmx/nested.c b/arch/x86/kvm/vmx/nested.c
> index 6058a65a6ede..f1dff1ebaccb 100644
> --- a/arch/x86/kvm/vmx/nested.c
> +++ b/arch/x86/kvm/vmx/nested.c
> @@ -3354,8 +3354,9 @@ enum nvmx_vmentry_status nested_vmx_enter_non_root_mode(struct kvm_vcpu *vcpu,
>  	}
>  
>  	enter_guest_mode(vcpu);
> -	if (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETTING)
> -		vcpu->arch.tsc_offset += vmcs12->tsc_offset;
> +
> +	kvm_set_02_tsc_offset(vcpu);
> +	kvm_set_02_tsc_multiplier(vcpu);

Please move this code into prepare_vmcs02() to co-locate it with the relevant
vmcs02 logic.  If there's something in prepare_vmcs02() that consumes
vcpu->arch.tsc_offset() other than the obvious VMWRITE, I vote to move things
around to fix that rather than create this weird split.

>  	if (prepare_vmcs02(vcpu, vmcs12, &entry_failure_code)) {
>  		exit_reason.basic = EXIT_REASON_INVALID_STATE;
> @@ -4463,8 +4464,12 @@ void nested_vmx_vmexit(struct kvm_vcpu *vcpu, u32 vm_exit_reason,
>  	if (nested_cpu_has_preemption_timer(vmcs12))
>  		hrtimer_cancel(&to_vmx(vcpu)->nested.preemption_timer);
>  
> -	if (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETTING)
> -		vcpu->arch.tsc_offset -= vmcs12->tsc_offset;
> +	if (vmcs12->cpu_based_vm_exec_control & CPU_BASED_USE_TSC_OFFSETTING) {
> +		vcpu->arch.tsc_offset = vcpu->arch.l1_tsc_offset;
> +
> +		if (vmcs12->secondary_vm_exec_control & SECONDARY_EXEC_TSC_SCALING)
> +			vcpu->arch.tsc_scaling_ratio = vcpu->arch.l1_tsc_scaling_ratio;
> +	}
>  
>  	if (likely(!vmx->fail)) {
>  		sync_vmcs02_to_vmcs12(vcpu, vmcs12);
> -- 
> 2.17.1
>