Subject: Re: [PATCHv4 2/2] vhost_net: a kernel-level virtio server

From: Rusty Russell <[email-masked]>
Date: Tue, 25 Aug 2009 21:40:40 +0930

On Thu, 20 Aug 2009 12:33:09 am Michael S. Tsirkin wrote:
> What it is: vhost net is a character device that can be used to reduce
> the number of system calls involved in virtio networking.
> Existing virtio net code is used in the guest without modification.

...

> +config VHOST_NET
> +	tristate "Host kernel accelerator for virtio net"
> +	depends on NET && EVENTFD
> +	---help---
> +	  This kernel module can be loaded in host kernel to accelerate
> +	  guest networking with virtio_net. Not to be confused with virtio_net
> +	  module itself which needs to be loaded in guest kernel.
> +
> +	  To compile this driver as a module, choose M here: the module will
> +	  be called vhost_net.

Just want to note that the patch explanation and the Kconfig help text are
exceptional examples of reader-oriented text.  Nice!

> +		/* Sanity check */
> +		if (vq->iov->iov_len != sizeof(struct virtio_net_hdr)) {
> +			vq_err(vq, "Unexpected header len for TX: "
> +			       "%ld expected %zd\n", vq->iov->iov_len,
> +			       sizeof(struct virtio_net_hdr));
> +			break;
> +		}

OK, this check, which is in the qemu version, is *wrong*.  There should be
no assumption on sg boundaries.  For example, the guest should be able to
put the virtio_net_hdr in the front of the skbuf data if there is room.

You should try to explicitly "consume" sizeof(struct virtio_net_hdr) of the
iov, and if fail, do this message.  You can skip the out <= 1 test then,
too.

Anyway, I really prefer vq->iov[0]. to vq->iov-> here.

> +		/* Sanity check */
> +		if (vq->iov->iov_len != sizeof(struct virtio_net_hdr)) {
> +			vq_err(vq, "Unexpected header len for RX: "
> +			       "%ld expected %zd\n",
> +			       vq->iov->iov_len, sizeof(struct virtio_net_hdr));
> +			break;

Here too.

> +	u32 __user *featurep = argp;
> +	int __user *fdp = argp;
> +	u32 features;
> +	int fd, r;
> +	switch (ioctl) {
> +	case VHOST_NET_SET_SOCKET:
> +		r = get_user(fd, fdp);
> +		if (r < 0)
> +			return r;
> +		return vhost_net_set_socket(n, fd);
> +	case VHOST_GET_FEATURES:
> +		/* No features for now */
> +		features = 0;
> +		return put_user(features, featurep);

We may well get more than 32 feature bits, at least for virtio_net, which will
force us to do some trickery in virtio_pci.  I'd like to avoid that here,
though it's kind of ugly.  We'd need VHOST_GET_FEATURES (and ACK) to take a
struct like:

	u32 feature_size;
	u32 features[];

> +int vhost_net_init(void)

static?

> +void vhost_net_exit(void)

static?

> +/* Start polling a file. We add ourselves to file's wait queue. The user must
> + * keep a reference to a file until after vhost_poll_stop is called. */

I experienced minor confusion from the comments in this file.  Where you said
"user" I think "caller".  No biggie though.

> +	memory->nregions = 2;
> +	memory->regions[0].guest_phys_addr = 1;
> +	memory->regions[0].userspace_addr = 1;
> +	memory->regions[0].memory_size = ~0ULL;
> +	memory->regions[1].guest_phys_addr = 0;
> +	memory->regions[1].userspace_addr = 0;
> +	memory->regions[1].memory_size = 1;

Not sure I understand why there are two regions to start?

> +	case VHOST_SET_VRING_BASE:
> +		r = copy_from_user(&s, argp, sizeof s);
> +		if (r < 0)
> +			break;
> +		if (s.num > 0xffff) {
> +			r = -EINVAL;
> +			break;
> +		}
> +		vq->last_avail_idx = s.num;
> +		break;
> +	case VHOST_GET_VRING_BASE:
> +		s.index = idx;
> +		s.num = vq->last_avail_idx;
> +		r = copy_to_user(argp, &s, sizeof s);
> +		break;

Ah, this is my fault.  I didn't expose the last_avail_idx in the ring
because the other side doesn't need it; but without it the ring state is not
fully observable from outside (no external save / restore!).

I have a patch which published these indices (we have room), see:
	http://ozlabs.org/~rusty/kernel/rr-2009-08-12-1/virtio:ring-publish-indices.patch

Perhaps we should use that mechanism instead?  We don't actually have to
offer the feature (we don't care about the guest state), but it's nice as
documentation.  I've been waiting for an excuse to use that patch.

> +long vhost_dev_ioctl(struct vhost_dev *d, unsigned int ioctl, unsigned long arg)
> +{
> +	void __user *argp = (void __user *)arg;
> +	long r;
> +
> +	mutex_lock(&d->mutex);
> +	if (ioctl == VHOST_SET_OWNER) {
> +		r = vhost_dev_set_owner(d);
> +		goto done;
> +	}
> +
> +	r = vhost_dev_check_owner(d);

You can do a VHOST_SET_OWNER without being the owner?  So really,
the -EPERM from all the vhost_dev_check_owner() is not a security thing,
but a "I don't think you mean to do that" thing?

If so, a comment above it might help?

> +static const struct vhost_memory_region *find_region(struct vhost_memory *mem,
> +						     __u64 addr, __u32 len)
> +{
> +	struct vhost_memory_region *reg;
> +	int i;
> +	/* linear search is not brilliant, but we really have on the order of 6
> +	 * regions in practice */

Ah, you actually mean "this code has been carefully cache-optimized for the
common case" :)

> +/* FIXME: this does not handle a region that spans multiple
> + * address/len pairs */
> +int translate_desc(struct vhost_dev *dev, u64 addr, u32 len,
> +		   struct iovec iov[], int iov_count, int iov_size,
> +		   unsigned *num)
> +{
> +	const struct vhost_memory_region *reg;
> +	struct vhost_memory *mem;
> +	struct iovec *_iov;
> +	u64 s = 0;
> +	int ret = 0;

Would this be neater if it returned the num iovecs used?  And offsetting
iov in the caller, rather than passing iov_count?

> +	/* If their number is silly, that's a fatal mistake. */
> +	if (head >= vq->num) {
> +		vq_err(vq, "Guest says index %u > %u is available",
> +		       head, vq->num);
> +		return vq->num;
> +	}

Not a fatal mistake in this code/

> +	vq->inflight++;

vq->inflight was a brain fart in the old lguest code.  See
commit ebf9a5a99c1a464afe0b4dfa64416fc8b273bc5c.

Also, see other fixes to the lguest launcher since then which might
be relevant to this code:
	lguest: get more serious about wmb() in example Launcher code
	lguest: clean up length-used value in example launcher

> +	/* If they don't want an interrupt, don't send one, unless empty. */
> +	if ((flags & VRING_AVAIL_F_NO_INTERRUPT) && vq->inflight)
> +		return;

And I wouldn't support notify on empty at all, TBH.  It should
definitely be conditional on the guest accepting the NOTIFY_ON_EMPTY
feature.

> +/* The virtqueue structure describes a queue attached to a device. */
> +struct vhost_virtqueue {
...
> +} ____cacheline_aligned;

Really?  I'd want to see numbers on this one.  False sharing vs. more cache
utilization.

> +#define vq_err(vq, fmt, ...) do {                                  \
> +		printk(KERN_ERR pr_fmt(fmt), ##__VA_ARGS__);       \
> +		if ((vq)->error_ctx)                               \
> +				eventfd_signal((vq)->error_ctx, 1);\
> +	} while (0)

Mmm... guests should not be able to create unlimited printks in the host.

But really, nothing major to object to in here...

Thanks!
Rusty.