On Thu, Aug 01, 2019 at 12:47:54PM +0200, Stefano Garzarella wrote:
> On Tue, Jul 30, 2019 at 04:42:25PM -0400, Michael S. Tsirkin wrote:
> > On Tue, Jul 30, 2019 at 11:35:39AM +0200, Stefano Garzarella wrote:
>
> (...)
>
> > >
> > > The problem here is the compatibility. Before this series virtio-vsock
> > > and vhost-vsock modules had the RX buffer size hard-coded
> > > (VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE = 4K). So, if we send a buffer smaller
> > > of 4K, there might be issues.
> >
> > Shouldn't be if they are following the spec. If not let's fix
> > the broken parts.
> >
> > >
> > > Maybe it is the time to add add 'features' to virtio-vsock device.
> > >
> > > Thanks,
> > > Stefano
> >
> > Why would a remote care about buffer sizes?
> >
> > Let's first see what the issues are. If they exist
> > we can either fix the bugs, or code the bug as a feature in spec.
> >
>
> The vhost_transport '.stream_enqueue' callback
> [virtio_transport_stream_enqueue()] calls the virtio_transport_send_pkt_info(),
> passing the user message. This function allocates a new packet, copying
> the user message, but (before this series) it limits the packet size to
> the VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE (4K):
>
> static int virtio_transport_send_pkt_info(struct vsock_sock *vsk,
> struct virtio_vsock_pkt_info *info)
> {
> ...
> /* we can send less than pkt_len bytes */
> if (pkt_len > VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE)
> pkt_len = VIRTIO_VSOCK_DEFAULT_RX_BUF_SIZE;
>
> /* virtio_transport_get_credit might return less than pkt_len credit */
> pkt_len = virtio_transport_get_credit(vvs, pkt_len);
>
> /* Do not send zero length OP_RW pkt */
> if (pkt_len == 0 && info->op == VIRTIO_VSOCK_OP_RW)
> return pkt_len;
> ...
> }
>
> then it queues the packet for the TX worker calling .send_pkt()
> [vhost_transport_send_pkt() in the vhost_transport case]
>
> The main function executed by the TX worker is
> vhost_transport_do_send_pkt() that picks up a buffer from the virtqueue
> and it tries to copy the packet (up to 4K) on it. If the buffer
> allocated from the guest will be smaller then 4K, I think here it will
> be discarded with an error:
>
> static void
> vhost_transport_do_send_pkt(struct vhost_vsock *vsock,
> struct vhost_virtqueue *vq)
> {
> ...
> nbytes = copy_to_iter(pkt->buf, pkt->len, &iov_iter);
isn't pck len the actual length though?
> if (nbytes != pkt->len) {
> virtio_transport_free_pkt(pkt);
> vq_err(vq, "Faulted on copying pkt buf\n");
> break;
> }
> ...
> }
>
>
> This series changes this behavior since now we will split the packet in
> vhost_transport_do_send_pkt() depending on the buffer found in the
> virtqueue.
>
> We didn't change the buffer size in this series, so we still backward
> compatible, but if we will use buffers smaller than 4K, we should
> encounter the error described above.
>
> How do you suggest we proceed if we want to change the buffer size?
> Maybe adding a feature to "support any buffer size"?
>
> Thanks,
> Stefano
