linux-kernel - Re: [PATCH v2 11/11] KVM: nSVM: implement support for nested VNMI

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Message-ID: <Y9m15P8xQ2dxvIzd@google.com>
Date:   Wed, 1 Feb 2023 00:44:20 +0000
From:   Sean Christopherson <seanjc@...gle.com>
To:     Maxim Levitsky <mlevitsk@...hat.com>
Cc:     kvm@...r.kernel.org, Sandipan Das <sandipan.das@....com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        Jim Mattson <jmattson@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Borislav Petkov <bp@...en8.de>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Josh Poimboeuf <jpoimboe@...nel.org>,
        Daniel Sneddon <daniel.sneddon@...ux.intel.com>,
        Jiaxi Chen <jiaxi.chen@...ux.intel.com>,
        Babu Moger <babu.moger@....com>, linux-kernel@...r.kernel.org,
        Jing Liu <jing2.liu@...el.com>,
        Wyes Karny <wyes.karny@....com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Santosh Shukla <santosh.shukla@....com>
Subject: Re: [PATCH v2 11/11] KVM: nSVM: implement support for nested VNMI

On Tue, Nov 29, 2022, Maxim Levitsky wrote:
> This patch allows L1 to use vNMI to accelerate its injection
> of NMIs to L2 by passing through vNMI int_ctl bits from vmcb12
> to/from vmcb02.
> 
> While L2 runs, L1's vNMI is inhibited, and L1's NMIs are injected
> normally.

Same feedback on stating the change as a command instead of describing the net
effects.

> In order to support nested VNMI requires saving and restoring the VNMI
> bits during nested entry and exit.

Again, avoid saving+restoring.  And it's not just for terminology, it's not a
true save/restore, e.g. a pending vNMI for L1 needs to be recognized and trigger
a nested VM-Exit.  I.e. KVM can't simply stash the state and restore it later,
KVM needs to actively process the pending NMI.

> In case of L1 and L2 both using VNMI- Copy VNMI bits from vmcb12 to
> vmcb02 during entry and vice-versa during exit.
> And in case of L1 uses VNMI and L2 doesn't- Copy VNMI bits from vmcb01 to
> vmcb02 during entry and vice-versa during exit.
> 
> Tested with the KVM-unit-test and Nested Guest scenario.
> 
> 
> Signed-off-by: Santosh Shukla <santosh.shukla@....com>
> Signed-off-by: Maxim Levitsky <mlevitsk@...hat.com>

Same SoB issues.

> ---
>  arch/x86/kvm/svm/nested.c | 13 ++++++++++++-
>  arch/x86/kvm/svm/svm.c    |  5 +++++
>  arch/x86/kvm/svm/svm.h    |  6 ++++++
>  3 files changed, 23 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kvm/svm/nested.c b/arch/x86/kvm/svm/nested.c
> index 5bea672bf8b12d..81346665058e26 100644
> --- a/arch/x86/kvm/svm/nested.c
> +++ b/arch/x86/kvm/svm/nested.c
> @@ -278,6 +278,11 @@ static bool __nested_vmcb_check_controls(struct kvm_vcpu *vcpu,
>  	if (CC(!nested_svm_check_tlb_ctl(vcpu, control->tlb_ctl)))
>  		return false;
>  
> +	if (CC((control->int_ctl & V_NMI_ENABLE) &&
> +		!vmcb12_is_intercept(control, INTERCEPT_NMI))) {

Align indentation.

	if (CC((control->int_ctl & V_NMI_ENABLE) &&
	       !vmcb12_is_intercept(control, INTERCEPT_NMI))) {
		return false;
	}

> +		return false;
> +	}
> +
>  	return true;
>  }
>  
> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
> index 0b7e1790fadde1..8fb2085188c5ac 100644
> --- a/arch/x86/kvm/svm/svm.h
> +++ b/arch/x86/kvm/svm/svm.h
> @@ -271,6 +271,7 @@ struct vcpu_svm {
>  	bool pause_filter_enabled         : 1;
>  	bool pause_threshold_enabled      : 1;
>  	bool vgif_enabled                 : 1;
> +	bool vnmi_enabled                 : 1;
>  
>  	u32 ldr_reg;
>  	u32 dfr_reg;
> @@ -545,6 +546,11 @@ static inline bool nested_npt_enabled(struct vcpu_svm *svm)
>  	return svm->nested.ctl.nested_ctl & SVM_NESTED_CTL_NP_ENABLE;
>  }
>  
> +static inline bool nested_vnmi_enabled(struct vcpu_svm *svm)
> +{
> +	return svm->vnmi_enabled && (svm->nested.ctl.int_ctl & V_NMI_ENABLE);

Gah, the "nested" flags in vcpu_svm are super confusing.  I initially read this
as "if vNMI is enabled in L1 and vmcb12".  

I have a series that I originally prepped for the architectural LBRs series that
will allow turning this into

	return guest_can_use(vcpu, X86_FEATURE_VNMI) &&
	       (svm->nested.ctl.int_ctl & V_NMI_ENABLE);

I'll get that series posted.

Nothing to do on your end, just an FYI.  I'll sort out conflicts if/when they happen.