Re: [PATCHv2 3/7] KVM: SVM: Add VNMI support in get/set_nmi_mask

["Shukla, Santosh" <santosh.shukla@amd.com>]

On 7/10/2022 9:45 PM, Maxim Levitsky wrote:
> On Sat, 2022-07-09 at 19:12 +0530, Santosh Shukla wrote:
>> VMCB intr_ctrl bit12 (V_NMI_MASK) is set by the processor when handling
>> NMI in guest and is cleared after the NMI is handled. Treat V_NMI_MASK as
>> read-only in the hypervisor and do not populate set accessors.
>>
>> Adding API(get_vnmi_vmcb) in order to return the correct vmcb for L1 or
>> L2.
>>
>> Signed-off-by: Santosh Shukla <santosh.shukla@amd.com>
>> ---
>> v2:
>> - Added get_vnmi_vmcb API to return vmcb for l1 and l2.
>> - Use get_vnmi_vmcb to get correct vmcb in func -
>>   is_vnmi_enabled/_mask_set()
>> - removed vnmi check from is_vnmi_enabled() func.
>>
>>  arch/x86/kvm/svm/svm.c | 12 ++++++++++--
>>  arch/x86/kvm/svm/svm.h | 32 ++++++++++++++++++++++++++++++++
>>  2 files changed, 42 insertions(+), 2 deletions(-)
>>
>> diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
>> index baaf35be36e5..3574e804d757 100644
>> --- a/arch/x86/kvm/svm/svm.c
>> +++ b/arch/x86/kvm/svm/svm.c
>> @@ -198,7 +198,7 @@ module_param(dump_invalid_vmcb, bool, 0644);
>>  bool intercept_smi = true;
>>  module_param(intercept_smi, bool, 0444);
>>  
>> -static bool vnmi;
>> +bool vnmi = true;
>>  module_param(vnmi, bool, 0444);
>>  
>>  static bool svm_gp_erratum_intercept = true;
>> @@ -3503,13 +3503,21 @@ static int svm_nmi_allowed(struct kvm_vcpu *vcpu, bool for_injection)
>>  
>>  static bool svm_get_nmi_mask(struct kvm_vcpu *vcpu)
>>  {
>> -	return !!(vcpu->arch.hflags & HF_NMI_MASK);
>> +	struct vcpu_svm *svm = to_svm(vcpu);
>> +
>> +	if (is_vnmi_enabled(svm))
>> +		return is_vnmi_mask_set(svm);
>> +	else
>> +		return !!(vcpu->arch.hflags & HF_NMI_MASK);
>>  }
>>  
>>  static void svm_set_nmi_mask(struct kvm_vcpu *vcpu, bool masked)
>>  {
>>  	struct vcpu_svm *svm = to_svm(vcpu);
>>  
>> +	if (is_vnmi_enabled(svm))
>> +		return;
>> +
>>  	if (masked) {
>>  		vcpu->arch.hflags |= HF_NMI_MASK;
>>  		if (!sev_es_guest(vcpu->kvm))
>> diff --git a/arch/x86/kvm/svm/svm.h b/arch/x86/kvm/svm/svm.h
>> index 9223ac100ef5..f36e30df6202 100644
>> --- a/arch/x86/kvm/svm/svm.h
>> +++ b/arch/x86/kvm/svm/svm.h
>> @@ -35,6 +35,7 @@ extern u32 msrpm_offsets[MSRPM_OFFSETS] __read_mostly;
>>  extern bool npt_enabled;
>>  extern int vgif;
>>  extern bool intercept_smi;
>> +extern bool vnmi;
>>  
>>  /*
>>   * Clean bits in VMCB.
>> @@ -509,6 +510,37 @@ static inline bool nested_npt_enabled(struct vcpu_svm *svm)
>>  	return svm->nested.ctl.nested_ctl & SVM_NESTED_CTL_NP_ENABLE;
>>  }
>>  
>> +static inline struct vmcb *get_vnmi_vmcb(struct vcpu_svm *svm)
>> +{
>> +	if (!vnmi)
>> +		return NULL;
>> +
>> +	if (is_guest_mode(&svm->vcpu))
>> +		return svm->nested.vmcb02.ptr;
>> +	else
>> +		return svm->vmcb01.ptr;
>> +}
> 
> This is better but still not enough to support nesting:
> 
> 
> Let me explain the cases that we need to cover:
> 
> 
> 1. non nested case, vmcb01 has all the VNMI settings,
> and I think it should work, but need to review the patches again.
> 
> 
> 
> 2. L1 uses vNMI, L2 doesn't use vNMI (nested_vnmi_enabled() == false).
> 
>   In this case, vNMI settings just need to be copied from vmcb01 to vmcb02
>   and vise versa during nested entry and exit.
> 
> 
>   This means that nested_vmcb02_prepare_control in this case should copy
>   all 3 bits from vmcb01 to vmcb02, and vise versa nested_svm_vmexit
>   should copy them back.
> 
>   Currently I see no indication of this being done in this patch series.
> 

Yes, Thanks for pointing out, in v3 series.

>   vmcb02 should indeed be used to read vnmi bits (like done above).
> 
> 
> 3. L1 uses vNMI, L2 uses vNMI:
> 
>   - First of all in this case all 3 vNMI bits should be copied from vmcb12
>     to vmcb02 on nested entry and back on nested VM exit.
> 
>     I *think* this is done correctly in the patch 6, but I need to check again.
> 
>  
>   - Second issue, depends on vNMI spec which we still don't have, and it
>     relates to the fact on what to do if NMIs are not intercepted by
>     the (nested) hypervisor, and L0 wants to inject an NMI
> 
>     (from L1 point of view it means that a 'real' NMI is about to be
>     received while L2 is running).
> 
> 
>     - If VNMI is not allowed to be enabled when NMIs are not intercepted,
>       (vast majority of nested hypervisors will want to intercept real NMIs)
>       then everything is fine -
> 
>       this means that if nested vNMI is enabled, then L1 will have
>       to intercept 'real' NMIs, and thus L0 would be able to always
>       inject 'real' NMIs while L2 is running by doing a VM exit to L1 without
>       touching any vNMI state.
> 
Yes. Enabling NMI virtualization requires the NMI intercept bit to be set.

>     - If the vNMI spec states that if vNMI is enabled, real NMIs
>       are not intercepted and a real NMI is arriving, then the CPU
>       will use vNMI state to handle it (that is it will set the 'pending'
>       bit, then check if 'masked' bit is set, and if not, move pending to masked
>       and deliver NMI to L2, in this case, it is indeed right to use vmcb02
>       and keep on using VNMI for NMIs that are directed to L1,
>       but I highly doubt that this is the case.
> 
> 
No.

>     - Most likely case - vNMI is allowed without NMI intercept,
>       and real NMI does't consult the vNMI bits, but rather uses 'hosts'
>       NMI masking. IRET doesn't affect host's NMI' masking as well.
> 
>

No.

Thanks,
Santosh
 
>       In this case, when L0 wants to inject NMI to a nested guest
>       that has vNMI enabled, and doesn't intercept NMIs, it
>       has to:
> 
>       - still consult the vNMI pending/masked bits of *vmcb01*,
>         to know if it can inject a NMI
> 
>       - if it can inject it, it should update *manually* the pending/masked bits
>         of vmcb01 as well, so that L1's vNMI the state remains consistent.
> 
>       - inject the NMI to L2, in the old fashioned way with EVENTINJ,
> 	or open NMI window by intercepting IRET if NMI is masked.
> 
> 
> Best regards,
> 	Maxim Levitsky
> 
> 
> 
> 
>> +
>> +static inline bool is_vnmi_enabled(struct vcpu_svm *svm)
>> +{
>> +	struct vmcb *vmcb = get_vnmi_vmcb(svm);
>> +
>> +	if (vmcb)
>> +		return !!(vmcb->control.int_ctl & V_NMI_ENABLE);
>> +	else
>> +		return false;
>> +}
>> +
>> +static inline bool is_vnmi_mask_set(struct vcpu_svm *svm)
>> +{
>> +	struct vmcb *vmcb = get_vnmi_vmcb(svm);
>> +
>> +	if (vmcb)
>> +		return !!(vmcb->control.int_ctl & V_NMI_MASK);
>> +	else
>> +		return false;
>> +}
>> +
>>  /* svm.c */
>>  #define MSR_INVALID				0xffffffffU
>>  
> 
>