From mboxrd@z Thu Jan  1 00:00:00 1970
From: Rusty Russell <rusty@rustcorp.com.au>
Subject: Re: [PATCH] virtio_pci: fix macro exported in uapi
Date: Mon, 20 May 2013 09:38:25 +0930
Message-ID: <87r4h2zf5y.fsf@rustcorp.com.au>
References: <87wqqy8o2k.fsf@rustcorp.com.au>
	<20130516164446.GA32007@redhat.com>
	<32418.1368781063@warthog.procyon.org.uk>
Mime-Version: 1.0
Content-Type: text/plain; charset="us-ascii"
Content-Transfer-Encoding: 7bit
Cc: kvm@vger.kernel.org, "Michael S. Tsirkin" <mst@redhat.com>,
	linux-kernel@vger.kernel.org,
	virtualization@lists.linux-foundation.org, dhowells@redhat.com,
	Dave Jones <davej@redhat.com>, Paolo Bonzini <pbonzini@redhat.com>,
	Stephen Hemminger <shemminger@vyatta.com>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Thomas Gleixner <tglx@linutronix.de>
To: David Howells <dhowells@redhat.com>
Return-path: <virtualization-bounces@lists.linux-foundation.org>
In-Reply-To: <32418.1368781063@warthog.procyon.org.uk>
List-Unsubscribe: <https://lists.linuxfoundation.org/mailman/options/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=unsubscribe>
List-Archive: <http://lists.linuxfoundation.org/pipermail/virtualization/>
List-Post: <mailto:virtualization@lists.linux-foundation.org>
List-Help: <mailto:virtualization-request@lists.linux-foundation.org?subject=help>
List-Subscribe: <https://lists.linuxfoundation.org/mailman/listinfo/virtualization>,
	<mailto:virtualization-request@lists.linux-foundation.org?subject=subscribe>
Sender: virtualization-bounces@lists.linux-foundation.org
Errors-To: virtualization-bounces@lists.linux-foundation.org
List-Id: kvm.vger.kernel.org

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.