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

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Date: Tue, 11 Jun 2024 21:18:32 +0800
From: Binbin Wu <binbin.wu@...ux.intel.com>
To: Sean Christopherson <seanjc@...gle.com>, isaku.yamahata@...el.com
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 isaku.yamahata@...il.com, Paolo Bonzini <pbonzini@...hat.com>,
 erdemaktas@...gle.com, Sagi Shahar <sagis@...gle.com>,
 Kai Huang <kai.huang@...el.com>, chen.bo@...el.com, hang.yuan@...el.com,
 tina.zhang@...el.com
Subject: Re: [PATCH v19 116/130] KVM: TDX: Silently discard SMI request



On 4/19/2024 9:52 PM, Sean Christopherson wrote:
> On Mon, Feb 26, 2024, isaku.yamahata@...el.com wrote:
>> From: Isaku Yamahata <isaku.yamahata@...el.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@...el.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.
>