Re: [RFC PATCH 16/27] virtio: Expose virtqueue_alloc_element

From: Stefan Hajnoczi <stefanha@redhat.com>
To: Eugenio Perez Martin <eperezma@redhat.com>
Cc: kvm list <kvm@vger.kernel.org>,
	"Michael S. Tsirkin" <mst@redhat.com>,
	qemu-level <qemu-devel@nongnu.org>,
	Daniel Daly <dandaly0@gmail.com>,
	virtualization@lists.linux-foundation.org,
	Liran Alon <liralon@gmail.com>, Eli Cohen <eli@mellanox.com>,
	Nitin Shrivastav <nitin.shrivastav@broadcom.com>,
	Alex Barba <alex.barba@broadcom.com>,
	Christophe Fontaine <cfontain@redhat.com>,
	Lee Ballard <ballle98@gmail.com>,
	Lars Ganrot <lars.ganrot@gmail.com>,
	Rob Miller <rob.miller@broadcom.com>,
	Howard Cai <howard.cai@gmail.com>,
	Parav Pandit <parav@mellanox.com>, vm <vmireyno@marvell.com>,
	Salil Mehta <mehta.salil.lnk@gmail.com>,
	Stephen Finucane <stephenfin@redhat.com>,
	Xiao W Wang <xiao.w.wang@intel.com>,
	Sean Mooney <smooney@redhat.com>,
	Jim Harford <jim.harford@broadcom.com>,
	Dmytro Kazantsev <dmytro.kazantsev@gmail.com>,
	Siwei Liu <loseweigh@gmail.com>,
	Harpreet Singh Anand <hanand@xilinx.com>,
	Michael Lilja <ml@napatech.com>, Max Gurtovoy <maxgu14@gmail.com>
Subject: Re: [RFC PATCH 16/27] virtio: Expose virtqueue_alloc_element
Date: Thu, 10 Dec 2020 11:57:29 +0000	[thread overview]
Message-ID: <20201210115729.GI416119@stefanha-x1.localdomain> (raw)
In-Reply-To: <CAJaqyWdN7iudf8mDN4k4Fs9j1x+ztZARuBbinPHD3ZQSMH1pyQ@mail.gmail.com>

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On Wed, Dec 09, 2020 at 07:46:49PM +0100, Eugenio Perez Martin wrote:
> On Tue, Dec 8, 2020 at 9:26 AM Stefan Hajnoczi <stefanha@gmail.com> wrote:
> >
> > On Fri, Nov 20, 2020 at 07:50:54PM +0100, Eugenio Pérez wrote:
> > > Specify VirtQueueElement * as return type makes no harm at this moment.
> >
> > The reason for the void * return type is that C implicitly converts void
> > pointers to pointers of any type. The function takes a size_t sz
> > argument so it can allocate a object of user-defined size. The idea is
> > that the user's struct embeds a VirtQueueElement field. Changing the
> > return type to VirtQueueElement * means that callers may need to
> > explicitly cast to the user's struct type.
> >
> > It's a question of coding style but I think the void * return type
> > communicates what is going on better than VirtQueueElement *.
> 
> Right, what I meant with that is that nobody uses that feature, but I
> just re-check and I saw that contrib/vhost-user-blk actually uses it
> (not checked for more uses). I think it is better just to drop this
> commit.

contrib/vhost-user-blk doesn't use hw/virtio/virtio.c. The code is
similar and copy-pasted, but you are free to change this file without
affecting vontrib/vhost-user-blk :).

I still think it's clearer to make it obvious that this function
allocates an object of generic type or at least the change is purely a
question of style and probably not worth making.

Stefan

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