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.
