linux-kernel - Re: [PATCH 05/10] KVM: SVM: Require AP's "requested" SEV

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Message-ID: <7f7f15e2-7ed0-b5fd-eb19-4423b4dabc46@amd.com>
Date: Mon, 24 Feb 2025 15:46:00 -0600
From: Tom Lendacky <thomas.lendacky@....com>
To: Sean Christopherson <seanjc@...gle.com>,
 Paolo Bonzini <pbonzini@...hat.com>
Cc: kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
 Naveen N Rao <naveen@...nel.org>, Kim Phillips <kim.phillips@....com>,
 Alexey Kardashevskiy <aik@....com>
Subject: Re: [PATCH 05/10] KVM: SVM: Require AP's "requested" SEV_FEATURES to
 match KVM's view

On 2/18/25 19:27, Sean Christopherson wrote:
> When handling an "AP Create" event, return an error if the "requested" SEV
> features for the vCPU don't exactly match KVM's view of the VM-scoped
> features.  There is no known use case for heterogeneous SEV features across
> vCPUs, and while KVM can't actually enforce an exact match since the value
> in RAX isn't guaranteed to match what the guest shoved into the VMSA, KVM
> can at least avoid knowingly letting the guest run in an unsupported state.
> 
> E.g. if a VM is created with DebugSwap disabled, KVM will intercept #DBs
> and DRs for all vCPUs, even if an AP is "created" with DebugSwap enabled in
> its VMSA.
> 
> Note, the GHCB spec only "requires" that "AP use the same interrupt
> injection mechanism as the BSP", but given the disaster that is DebugSwap
> and SEV_FEATURES in general, it's safe to say that AMD didn't consider all
> possible complications with mismatching features between the BSP and APs.
> 
> Oppurtunistically fold the check into the relevant request flavors; the
> "request < AP_DESTROY" check is just a bizarre way of implementing the
> AP_CREATE_ON_INIT => AP_CREATE fallthrough.

s/Oppurtunistically/Opportunistically/

Yes, with the change in patch #4 to not kick vCPUs on AP creation
failure, that check can now be moved to the switch statement.

Reviewed-by: Tom Lendacky <thomas.lendacky@....com>

> 
> Fixes: e366f92ea99e ("KVM: SEV: Support SEV-SNP AP Creation NAE event")
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>
> ---
>  arch/x86/kvm/svm/sev.c | 23 ++++++++---------------
>  1 file changed, 8 insertions(+), 15 deletions(-)
> 
> diff --git a/arch/x86/kvm/svm/sev.c b/arch/x86/kvm/svm/sev.c
> index 07125b2cf0a6..8425198c5204 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3934,6 +3934,7 @@ void sev_snp_init_protected_guest_state(struct kvm_vcpu *vcpu)
>  
>  static int sev_snp_ap_creation(struct vcpu_svm *svm)
>  {
> +	struct kvm_sev_info *sev = to_kvm_sev_info(svm->vcpu.kvm);
>  	struct kvm_vcpu *vcpu = &svm->vcpu;
>  	struct kvm_vcpu *target_vcpu;
>  	struct vcpu_svm *target_svm;
> @@ -3965,26 +3966,18 @@ static int sev_snp_ap_creation(struct vcpu_svm *svm)
>  
>  	mutex_lock(&target_svm->sev_es.snp_vmsa_mutex);
>  
> -	/* Interrupt injection mode shouldn't change for AP creation */
> -	if (request < SVM_VMGEXIT_AP_DESTROY) {
> -		u64 sev_features;
> -
> -		sev_features = vcpu->arch.regs[VCPU_REGS_RAX];
> -		sev_features ^= to_kvm_sev_info(svm->vcpu.kvm)->vmsa_features;
> -
> -		if (sev_features & SVM_SEV_FEAT_INT_INJ_MODES) {
> -			vcpu_unimpl(vcpu, "vmgexit: invalid AP injection mode [%#lx] from guest\n",
> -				    vcpu->arch.regs[VCPU_REGS_RAX]);
> -			ret = -EINVAL;
> -			goto out;
> -		}
> -	}
> -
>  	switch (request) {
>  	case SVM_VMGEXIT_AP_CREATE_ON_INIT:
>  		kick = false;
>  		fallthrough;
>  	case SVM_VMGEXIT_AP_CREATE:
> +		if (vcpu->arch.regs[VCPU_REGS_RAX] != sev->vmsa_features) {
> +			vcpu_unimpl(vcpu, "vmgexit: mismatched AP sev_features [%#lx] != [%#llx] from guest\n",
> +				    vcpu->arch.regs[VCPU_REGS_RAX], sev->vmsa_features);
> +			ret = -EINVAL;
> +			goto out;
> +		}
> +
>  		if (!page_address_valid(vcpu, svm->vmcb->control.exit_info_2)) {
>  			vcpu_unimpl(vcpu, "vmgexit: invalid AP VMSA address [%#llx] from guest\n",
>  				    svm->vmcb->control.exit_info_2);