Re: [PATCH v12 11/20] KVM: xen: allow shared_info to be mapped by fixed HVA

[Sean Christopherson <seanjc@google.com>]

On Thu, Feb 08, 2024, David Woodhouse wrote:
> Using anything except NULL as the "no value" value doesn't make sense
> to me. It violates the principle of least surprise and would be a
> really bad API.

I'm a-ok with using '0'.  My only request is to check for "!hva" as opposed to
"hva == 0", both because that's preferred kernel style, and because it better
conveys that it's really checking for !NULL as opposed to address '0'.

Side topic, I think the code will end up in a more readable state if the GFN vs.
HVA sub-commands are handled in separate case statements, especially if/when
xen_lock goes away.  E.g. something like this:

	case KVM_XEN_ATTR_TYPE_SHARED_INFO: {
		int idx;

		if (data->u.shared_info.gfn == KVM_XEN_INVALID_GFN) {
			kvm_gpc_deactivate(&kvm->arch.xen.shinfo_cache);
			r = 0;
			break;
		}

		idx = srcu_read_lock(&kvm->srcu);
		r = kvm_gpc_activate(&kvm->arch.xen.shinfo_cache,
				     gfn_to_gpa(data->u.shared_info.gfn), PAGE_SIZE);
		if (!r && kvm->arch.xen.shinfo_cache.active)
			r = kvm_xen_shared_info_init(kvm);
		srcu_read_unlock(&kvm->srcu, idx);
		break;
	}

	case KVM_XEN_ATTR_TYPE_SHARED_INFO_HVA: {
		unsigned long hva = data->u.shared_info.hva;

		if (hva != untagged_addr(hva) || !access_ok((void __user *)hva) ||
		    !PAGE_ALIGNED(hva)) {
			r = -EINVAL;
			break;
		}

		if (!hva) {
			kvm_gpc_deactivate(&kvm->arch.xen.shinfo_cache);
			r = 0;
			break;
		}
		r = kvm_gpc_activate_hva(&kvm->arch.xen.shinfo_cache, hva, PAGE_SIZE);
		if (!r && kvm->arch.xen.shinfo_cache.active)
			r = kvm_xen_shared_info_init(kvm);
		break;
	}

Side topic #2, the above requires that __kvm_gpc_refresh() not grab kvm_memslots()
in the "using an hva" path, but I think that'd actually be a good thing as it
would make it a bit more clear that using an hva bypasses memslots by design.