linux-kernel - Re: [PATCH] virtio_pci: fix macro exported in uapi

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Date:	Mon, 20 May 2013 09:38:25 +0930
From:	Rusty Russell <rusty@...tcorp.com.au>
To:	David Howells <dhowells@...hat.com>
Cc:	dhowells@...hat.com, "Michael S. Tsirkin" <mst@...hat.com>,
	kvm@...r.kernel.org, Stephen Hemminger <shemminger@...tta.com>,
	Jason Wang <jasowang@...hat.com>,
	Paolo Bonzini <pbonzini@...hat.com>,
	"Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
	Dave Jones <davej@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	virtualization@...ts.linux-foundation.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] virtio_pci: fix macro exported in uapi

David Howells <dhowells@...hat.com> writes:
> Rusty Russell <rusty@...tcorp.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.
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