Re: [PATCH v19 116/130] KVM: TDX: Silently discard SMI request

Binbin Wu
2024-06-11T13:18:38Z

On 4/19/2024 9:52 PM, Sean Christopherson wrote:
> On Mon, Feb 26, 2024, isaku.yamahata@intel.com wrote:
>> From: Isaku Yamahata <isaku.yamahata@intel.com>
>>
>> TDX doesn't support system-management mode (SMM) and system-management
>> interrupt (SMI) in guest TDs. Because guest state (vcpu state, memory
>> state) is protected, it must go through the TDX module APIs to change guest
>> state, injecting SMI and changing vcpu mode into SMM. The TDX module
>> doesn't provide a way for VMM to inject SMI into guest TD and a way for VMM
>> to switch guest vcpu mode into SMM.
>>
>> We have two options in KVM when handling SMM or SMI in the guest TD or the
>> device model (e.g. QEMU): 1) silently ignore the request or 2) return a
>> meaningful error.
>>
>> For simplicity, we implemented the option 1).
>>
>> Signed-off-by: Isaku Yamahata <isaku.yamahata@intel.com>
>> ---
>> arch/x86/kvm/smm.h | 7 +++++-
>> arch/x86/kvm/vmx/main.c | 45 ++++++++++++++++++++++++++++++++++----
>> arch/x86/kvm/vmx/tdx.c | 29 ++++++++++++++++++++++++
>> arch/x86/kvm/vmx/x86_ops.h | 12 ++++++++++
>> 4 files changed, 88 insertions(+), 5 deletions(-)
>>
>> diff --git a/arch/x86/kvm/smm.h b/arch/x86/kvm/smm.h
>> index a1cf2ac5bd78..bc77902f5c18 100644
>> --- a/arch/x86/kvm/smm.h
>> +++ b/arch/x86/kvm/smm.h
>> @@ -142,7 +142,12 @@ union kvm_smram {
>>
>> static inline int kvm_inject_smi(struct kvm_vcpu *vcpu)
>> {
>> - kvm_make_request(KVM_REQ_SMI, vcpu);
>> + /*
>> + * If SMM isn't supported (e.g. TDX), silently discard SMI request.
>> + * Assume that SMM supported = MSR_IA32_SMBASE supported.
>> + */
>> + if (static_call(kvm_x86_has_emulated_msr)(vcpu->kvm, MSR_IA32_SMBASE))
>> + kvm_make_request(KVM_REQ_SMI, vcpu);
>> return 0;
> No, just do what KVM already does for CONFIG_KVM_SMM=n, and return -ENOTTY. The
> *entire* point of have a return code is to handle setups that don't support SMM.
>
> if (!static_call(kvm_x86_has_emulated_msr)(vcpu->kvm, MSR_IA32_SMBASE)))
> return -ENOTTY;
>
> And with that, I would drop the comment, it's pretty darn clear what "assumption"
> is being made. In quotes because it's not an assumption, it's literally KVM's
> implementation.
>
> And then the changelog can say "do what KVM does for CONFIG_KVM_SMM=n" without
> having to explain why we decided to do something completely arbitrary for TDX.
>
>> }
>>
>> diff --git a/arch/x86/kvm/vmx/main.c b/arch/x86/kvm/vmx/main.c
>> index ed46e7e57c18..4f3b872cd401 100644
>> --- a/arch/x86/kvm/vmx/main.c
>> +++ b/arch/x86/kvm/vmx/main.c
>> @@ -283,6 +283,43 @@ static void vt_msr_filter_changed(struct kvm_vcpu *vcpu)
>> vmx_msr_filter_changed(vcpu);
>> }
>>
>> +#ifdef CONFIG_KVM_SMM
>> +static int vt_smi_allowed(struct kvm_vcpu *vcpu, bool for_injection)
>> +{
>> + if (is_td_vcpu(vcpu))
>> + return tdx_smi_allowed(vcpu, for_injection);
> Adding stubs for something that TDX will never support is silly. Bug the VM and
> return an error.
>
> if (KVM_BUG_ON(is_td_vcpu(vcpu)))
> return -EIO;

is_td_vcpu() is defined in tdx.h.
Do you mind using open code to check whether the VM is TD in vmx.c?
"vcpu->kvm->arch.vm_type == KVM_X86_TDX_VM"

>
> And I wouldn't even bother with vt_* wrappers, just put that right in vmx_*().
> Same thing for everything below.
>