Re: [PATCH] virtio_pci: fix macro exported in uapi

[Rusty Russell <rusty@rustcorp.com.au>]

David Howells <dhowells@redhat.com> writes:
> Rusty Russell <rusty@rustcorp.com.au> wrote:
>
>> Macro still isn't usable, because userspace can't know whether it's the
>> new or old.
>> 
>> We need to either remove it from UAPI, or rename it to
>> VIRTIO_PCI_CONFIG_OFF.
>
> Surely, if userspace is using it as is, you can't remove it, rename it or
> alter it?

The point of the patch is that it's unusable:

#define VIRTIO_PCI_CONFIG(dev)		((dev)->msix_enabled ? 24 : 20)

ie. it's accessing a member of the kernel's virtio_pci_dev structure.

In theory, userspace could have a structure with the same field and be
using it now, but that's unlikely (qemu certainly doesn't).  No harm no
foul.

In theory it's actually a useful macro, so we could expose it to
userspace, but we'd need to have a new name so userspace can #ifdef for
older headers...

Hope that clarifies,
Rusty.