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

[Maxim Levitsky]

On Mon, 2022-12-05 at 22:44 +0530, Santosh Shukla wrote:
> On 11/30/2022 1:07 AM, 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.
> > 
> > In order to support nested VNMI requires saving and restoring the VNMI
> > bits during nested entry and exit.
> > 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@xxxxxxx>
> > Signed-off-by: Maxim Levitsky <mlevitsk@xxxxxxxxxx>
> > ---
> >  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))) {
> > +		return false;
> > +	}
> > +
> >  	return true;
> >  }
> >  
> > @@ -427,6 +432,9 @@ void nested_sync_control_from_vmcb02(struct vcpu_svm *svm)
> >  	if (nested_vgif_enabled(svm))
> >  		mask |= V_GIF_MASK;
> >  
> > +	if (nested_vnmi_enabled(svm))
> > +		mask |= V_NMI_MASK | V_NMI_PENDING;
> > +
> >  	svm->nested.ctl.int_ctl        &= ~mask;
> >  	svm->nested.ctl.int_ctl        |= svm->vmcb->control.int_ctl & mask;
> >  }
> > @@ -682,8 +690,11 @@ static void nested_vmcb02_prepare_control(struct vcpu_svm *svm,
> >  	else
> >  		int_ctl_vmcb01_bits |= (V_GIF_MASK | V_GIF_ENABLE_MASK);
> >  
> > -	if (vnmi)
> > +	if (vnmi) {
> 
> To avoid above change, I think we should move nested bits from 10/11 to this i.e.. move function
> (nested_svm_save_vnmi and nested_svm_restore_vnmi) to this patch.
> 
> make sense?


This is done on purpose:

For each optional SVM feature there are two parts in regard to nesting.

First part is the nesting co-existance, meaning that KVM should still work
while a nested guest runs, and the second part is letting the nested hypervisor
use the feature.

First part is mandatory, as otherwise KVM will be broken while a nested
guest runs.

I squashed all of the vNMI support including nested co-existance in the patch 10,
and that includes the 'nested_svm_save_vnmi' and 'nested_svm_restore_vnmi'

Now this patch adds the actual nested vNMI, meaning that the nested hypervisor can
use it to speed up the delivery of NMI, it would like to inject to L2.

Best regards,
	Maxim Levitsky

> 
> Thanks,
> Santosh
> 
> >  		nested_svm_save_vnmi(svm);
> > +		if (nested_vnmi_enabled(svm))
> > +			int_ctl_vmcb12_bits |= (V_NMI_PENDING | V_NMI_ENABLE | V_NMI_MASK);
> > +	}
> >  
> >  	/* Copied from vmcb01.  msrpm_base can be overwritten later.  */
> >  	vmcb02->control.nested_ctl = vmcb01->control.nested_ctl;
> > diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
> > index bf10adcf3170a8..fb203f536d2f9b 100644
> > --- a/arch/x86/kvm/svm/svm.c
> > +++ b/arch/x86/kvm/svm/svm.c
> > @@ -4214,6 +4214,8 @@ static void svm_vcpu_after_set_cpuid(struct kvm_vcpu *vcpu)
> >  
> >  	svm->vgif_enabled = vgif && guest_cpuid_has(vcpu, X86_FEATURE_VGIF);
> >  
> > +	svm->vnmi_enabled = vnmi && guest_cpuid_has(vcpu, X86_FEATURE_AMD_VNMI);
> > +
> >  	svm_recalc_instruction_intercepts(vcpu, svm);
> >  
> >  	/* For sev guests, the memory encryption bit is not reserved in CR3.  */
> > @@ -4967,6 +4969,9 @@ static __init void svm_set_cpu_caps(void)
> >  		if (vgif)
> >  			kvm_cpu_cap_set(X86_FEATURE_VGIF);
> >  
> > +		if (vnmi)
> > +			kvm_cpu_cap_set(X86_FEATURE_AMD_VNMI);
> > +
> >  		/* Nested VM can receive #VMEXIT instead of triggering #GP */
> >  		kvm_cpu_cap_set(X86_FEATURE_SVME_ADDR_CHK);
> >  	}
> > 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);
> > +}
> > +
> >  static inline bool is_x2apic_msrpm_offset(u32 offset)
> >  {
> >  	/* 4 msrs per u8, and 4 u8 in u32 */