linux-kernel - Re: [PATCH v2 05/10] KVM: SVM: Require AP's "requested" SEV

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Message-ID: <4443bdf2-c8ea-4245-a23f-bb561c7e734e@amd.com>
Date: Thu, 27 Feb 2025 08:12:26 +0100
From: "Gupta, Pankaj" <pankaj.gupta@....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>,
 Tom Lendacky <thomas.lendacky@....com>, Alexey Kardashevskiy <aik@....com>
Subject: Re: [PATCH v2 05/10] KVM: SVM: Require AP's "requested" SEV_FEATURES
 to match KVM's view

On 2/27/2025 2:25 AM, 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.
> 
> Opportunistically 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.
> 
> Fixes: e366f92ea99e ("KVM: SEV: Support SEV-SNP AP Creation NAE event")
> Reviewed-by: Tom Lendacky <thomas.lendacky@....com>
> Signed-off-by: Sean Christopherson <seanjc@...gle.com>

Looks good. Even makes code simpler.

A minor query below.

Reviewed-by: Pankaj Gupta <pankaj.gupta@....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 9aad0dae3a80..bad5834ec143 100644
> --- a/arch/x86/kvm/svm/sev.c
> +++ b/arch/x86/kvm/svm/sev.c
> @@ -3932,6 +3932,7 @@ 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;
> @@ -3963,26 +3964,18 @@ 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) {

'SVM_SEV_FEAT_INT_INJ_MODES' would even be required in any future 
use-case, maybe?

Thanks,
Pankaj
> -			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);