Re: [PATCH v14 22/22] KVM: SEV: Provide support for SNP_EXTENDED_GUEST_REQUEST NAE event

[Sean Christopherson]

On Sun, Apr 21, 2024, Michael Roth wrote:
> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
> index 85099198a10f..6cf186ed8f66 100644
> --- a/Documentation/virt/kvm/api.rst
> +++ b/Documentation/virt/kvm/api.rst
> @@ -7066,6 +7066,7 @@ values in kvm_run even if the corresponding bit in kvm_dirty_regs is not set.
>  		struct kvm_user_vmgexit {
>  		#define KVM_USER_VMGEXIT_PSC_MSR	1
>  		#define KVM_USER_VMGEXIT_PSC		2
> +		#define KVM_USER_VMGEXIT_EXT_GUEST_REQ	3

Assuming we can't get massage this into a vendor agnostic exit, there's gotta be
a better name than EXT_GUEST_REQ, which is completely meaningless to me and probably
most other people that aren't intimately familar with the specs.  Request what?

>  			__u32 type; /* KVM_USER_VMGEXIT_* type */
>  			union {
>  				struct {
> @@ -3812,6 +3813,84 @@ static void snp_handle_guest_req(struct vcpu_svm *svm, gpa_t req_gpa, gpa_t resp
>  	ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, SNP_GUEST_ERR(vmm_ret, fw_err));
>  }
>  
> +static int snp_complete_ext_guest_req(struct kvm_vcpu *vcpu)
> +{
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +	struct vmcb_control_area *control;
> +	struct kvm *kvm = vcpu->kvm;
> +	sev_ret_code fw_err = 0;
> +	int vmm_ret;
> +
> +	vmm_ret = vcpu->run->vmgexit.ext_guest_req.ret;
> +	if (vmm_ret) {
> +		if (vmm_ret == SNP_GUEST_VMM_ERR_INVALID_LEN)
> +			vcpu->arch.regs[VCPU_REGS_RBX] =
> +				vcpu->run->vmgexit.ext_guest_req.data_npages;
> +		goto abort_request;
> +	}
> +
> +	control = &svm->vmcb->control;
> +
> +	/*
> +	 * To avoid the message sequence number getting out of sync between the
> +	 * actual value seen by firmware verses the value expected by the guest,
> +	 * make sure attestations can't get paused on the write-side at this
> +	 * point by holding the lock for the entire duration of the firmware
> +	 * request so that there is no situation where SNP_GUEST_VMM_ERR_BUSY
> +	 * would need to be returned after firmware sees the request.
> +	 */
> +	mutex_lock(&snp_pause_attestation_lock);

Why is this in KVM?  IIUC, KVM only needs to get involved to translate GFNs to
PFNs, the rest can go in the sev-dev driver, no?  The whole split is weird,
seemingly due to KVM "needing" to take this lock.  E.g. why is core kernel code
in arch/x86/virt/svm/sev.c at all dealing with attestation goo, when pretty much
all of the actual usage is (or can be) in sev-dev.c

> +
> +	if (__snp_transaction_is_stale(svm->snp_transaction_id))
> +		vmm_ret = SNP_GUEST_VMM_ERR_BUSY;
> +	else if (!__snp_handle_guest_req(kvm, control->exit_info_1,
> +					 control->exit_info_2, &fw_err))
> +		vmm_ret = SNP_GUEST_VMM_ERR_GENERIC;
> +
> +	mutex_unlock(&snp_pause_attestation_lock);
> +
> +abort_request:
> +	ghcb_set_sw_exit_info_2(svm->sev_es.ghcb, SNP_GUEST_ERR(vmm_ret, fw_err));
> +
> +	return 1; /* resume guest */
> +}
> +
> +static int snp_begin_ext_guest_req(struct kvm_vcpu *vcpu)
> +{
> +	int vmm_ret = SNP_GUEST_VMM_ERR_GENERIC;
> +	struct vcpu_svm *svm = to_svm(vcpu);
> +	unsigned long data_npages;
> +	sev_ret_code fw_err;
> +	gpa_t data_gpa;
> +
> +	if (!sev_snp_guest(vcpu->kvm))
> +		goto abort_request;
> +
> +	data_gpa = vcpu->arch.regs[VCPU_REGS_RAX];
> +	data_npages = vcpu->arch.regs[VCPU_REGS_RBX];
> +
> +	if (!IS_ALIGNED(data_gpa, PAGE_SIZE))
> +		goto abort_request;
> +
> +	svm->snp_transaction_id = snp_transaction_get_id();
> +	if (snp_transaction_is_stale(svm->snp_transaction_id)) {

Why bother?  I assume this is rare, so why not handle it on the backend, i.e.
after userspace does its thing?  Then KVM doesn't even have to care about
checking for stale IDs, I think?

None of this makes much sense to my eyes, e.g. AFAICT, the only thing that can
pause attestation is userspace, yet the kernel is responsible for tracking whether
or not a transaction is fresh?