Subject: Re: [PATCH 2/2] vhost_net: a kernel-level virtio server

From: "Michael S. Tsirkin" <[email-masked]>
Date: Wed, 12 Aug 2009 20:21:41 +0300

On Wed, Aug 12, 2009 at 07:03:22PM +0200, Arnd Bergmann wrote:
> On Monday 10 August 2009, Michael S. Tsirkin wrote:
> 
> > +struct workqueue_struct *vhost_workqueue;
> 
> [nitpicking] This could be static. 

Good catch. Thanks!

> > +/* The virtqueue structure describes a queue attached to a device. */
> > +struct vhost_virtqueue {
> > +	struct vhost_dev *dev;
> > +
> > +	/* The actual ring of buffers. */
> > +	struct mutex mutex;
> > +	unsigned int num;
> > +	struct vring_desc __user *desc;
> > +	struct vring_avail __user *avail;
> > +	struct vring_used __user *used;
> > +	struct file *kick;
> > +	struct file *call;
> > +	struct file *error;
> > +	struct eventfd_ctx *call_ctx;
> > +	struct eventfd_ctx *error_ctx;
> > +
> > +	struct vhost_poll poll;
> > +
> > +	/* The routine to call when the Guest pings us, or timeout. */
> > +	work_func_t handle_kick;
> > +
> > +	/* Last available index we saw. */
> > +	u16 last_avail_idx;
> > +
> > +	/* Last index we used. */
> > +	u16 last_used_idx;
> > +
> > +	/* Outstanding buffers */
> > +	unsigned int inflight;
> > +
> > +	/* Is this blocked? */
> > +	bool blocked;
> > +
> > +	struct iovec iov[VHOST_NET_MAX_SG];
> > +
> > +} ____cacheline_aligned;
> 
> We discussed this before, and I still think this could be directly derived
> from struct virtqueue, in the same way that vring_virtqueue is derived from
> struct virtqueue.

I prefer keeping it simple. Much of abstraction in virtio is due to the
fact that it needs to work on top of different hardware emulations:
lguest,kvm, possibly others in the future.  vhost is always working on
real hardware, using eventfd as the interface, so it does not need that.

> That would make it possible for simple device drivers
> to use the same driver in both host and guest,

I don't think so. For example, there's a callback field that gets
invoked in guest when buffers are consumed.  It could be overloaded to
mean "buffers are available" in host but you never handle both
situations in the same way, so what's the point?

> similar to how Ira Snyder used virtqueues to make virtio_net run
> between two hosts running the same code [1].
> Ideally, I guess you should be able to even make virtio_net work in the
> host if you do that, but that could bring other complexities.
> 
> 	Arnd <><
> 
> [1] http://lkml.org/lkml/2009/2/23/353

As I pointed out earlier, most code in virtio net is asymmetrical: guest
provides buffers, host consumes them.  Possibly, one could use virtio
rings in a symmetrical way, but support of existing guest virtio net
means there's almost no shared code.

-- 
MST