Re: [RFC PATCH] Add Hyperv extended hypercall support in KVM

[Sean Christopherson <seanjc@google.com>]

On Mon, Oct 24, 2022, Vitaly Kuznetsov wrote:
> While some 'extended' hypercalls may indeed need to be handled in KVM,
> there's no harm done in forwarding all unknown-to-KVM hypercalls to
> userspace. The only issue I envision is how would userspace discover
> which extended hypercalls are supported by KVM in case it (userspace) is
> responsible for handling HvExtCallQueryCapabilities call which returns
> the list of supported hypercalls. E.g. in case we decide to implement
> HvExtCallMemoryHeatHint in KVM, how are we going to communicate this to
> userspace?
> 
> Normally, VMM discovers the availability of Hyper-V features through
> KVM_GET_SUPPORTED_HV_CPUID but extended hypercalls are not listed in
> CPUID. This can be always be solved by adding new KVM CAPs of
> course. Alternatively, we can add a single
> "KVM_CAP_HYPERV_EXT_CALL_QUERY" which will just return the list of
> extended hypercalls supported by KVM (which Vipin's patch adds anyway to
> *set* the list instead).

AIUI, the TLFS uses a 64-bit mask to enumerate which extended hypercalls are
supported, so a single CAP should be a perfect fit.  And KVM can use the capability
to enumerate support for _and_ to allow userspace to enable in-kernel handling.  E.g.

check():
	case KVM_CAP_HYPERV_EXT_CALL:
		return KVM_SUPPORTED_HYPERV_EXT_CALL;


enable():

	case KVM_CAP_HYPERV_EXT_CALL:
		r = -EINVAL;
		if (mask & ~KVM_SUPPORTED_HYPERV_EXT_CALL)
			break;

		mutex_lock(&kvm->lock);
		if (!kvm->created_vcpus) {
			to_kvm_hv(kvm)->ext_call = cap->args[0];
			r = 0;
		}
		mutex_unlock(&kvm->lock);

kvm_hv_hypercall()


	case HV_EXT_CALL_QUERY_CAPABILITIES ... HV_EXT_CALL_MAX:
		if (unlikely(hc.fast)) {
			ret = HV_STATUS_INVALID_PARAMETER;
			break;
		}
		if (!(hc.code & to_kvm_hv(vcpu->kvm)->ext_call))
			goto hypercall_userspace_exit;

		ret = kvm_hv_ext_hypercall(...)
		break;


That maintains backwards compatibility with "exit on everything" as userspace
still needs to opt-in to having KVM handle specific hypercalls in-kernel, and it
provides the necessary knob for userspace to tell KVM which hypercalls should be
allowed, i.e. ensures KVM doesn't violate HV_EXT_CALL_QUERY_CAPABILITIES.