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Date:   Thu, 1 Aug 2019 09:21:15 -0400
From:   "Michael S. Tsirkin" <mst@redhat.com>
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     netdev@vger.kernel.org, linux-kernel@vger.kernel.org,
        Stefan Hajnoczi <stefanha@redhat.com>,
        "David S. Miller" <davem@davemloft.net>,
        virtualization@lists.linux-foundation.org,
        Jason Wang <jasowang@redhat.com>, kvm@vger.kernel.org
Subject: Re: [PATCH v4 1/5] vsock/virtio: limit the memory used per-socket
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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