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

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Message-ID: <a506d7cf3cde8c2b258fb6d31088030e2ce171ab.camel@amazon.com>
Date:   Wed, 19 May 2021 11:55:43 +0000
From:   "Stamatis, Ilias" <ilstam@...zon.com>
To:     "seanjc@...gle.com" <seanjc@...gle.com>
CC:     "kvm@...r.kernel.org" <kvm@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "jmattson@...gle.com" <jmattson@...gle.com>,
        "Woodhouse, David" <dwmw@...zon.co.uk>,
        "vkuznets@...hat.com" <vkuznets@...hat.com>,
        "joro@...tes.org" <joro@...tes.org>,
        "mtosatti@...hat.com" <mtosatti@...hat.com>,
        "zamsden@...il.com" <zamsden@...il.com>,
        "pbonzini@...hat.com" <pbonzini@...hat.com>,
        "mlevitsk@...hat.com" <mlevitsk@...hat.com>,
        "wanpengli@...cent.com" <wanpengli@...cent.com>
Subject: Re: [PATCH v2 08/10] KVM: VMX: Set the TSC offset and multiplier on nested
 entry and exit

On Wed, 2021-05-19 at 00:07 +0000, Sean Christopherson wrote:
> 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.
> 

Agreed. It makes more sense.

> >       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;
> > +     }

I guess these need to stay where they are though.

And thinking about it the two if conditions are unnecessary really.

Thanks for the reviews!

Ilias

> > 
> >       if (likely(!vmx->fail)) {
> >               sync_vmcs02_to_vmcs12(vcpu, vmcs12);
> > --
> > 2.17.1
> >