Re: [PATCH RFC net-next v4 6/8] virtio/vsock: support dgrams

[Stefano Garzarella]

On Tue, Jun 27, 2023 at 01:19:43AM +0000, Bobby Eshleman wrote:
> On Mon, Jun 26, 2023 at 05:03:15PM +0200, Stefano Garzarella wrote:
> > On Fri, Jun 23, 2023 at 04:37:55AM +0000, Bobby Eshleman wrote:
> > > On Thu, Jun 22, 2023 at 06:09:12PM +0200, Stefano Garzarella wrote:
> > > > On Sun, Jun 11, 2023 at 11:49:02PM +0300, Arseniy Krasnov wrote:
> > > > > Hello Bobby!
> > > > >
> > > > > On 10.06.2023 03:58, Bobby Eshleman wrote:
> > > > > > This commit adds support for datagrams over virtio/vsock.
> > > > > >
> > > > > > Message boundaries are preserved on a per-skb and per-vq entry basis.
> > > > >
> > > > > I'm a little bit confused about the following case: let vhost sends 4097 bytes
> > > > > datagram to the guest. Guest uses 4096 RX buffers in it's virtio queue, each
> > > > > buffer has attached empty skb to it. Vhost places first 4096 bytes to the first
> > > > > buffer of guests RX queue, and 1 last byte to the second buffer. Now IIUC guest
> > > > > has two skb in it rx queue, and user in guest wants to read data - does it read
> > > > > 4097 bytes, while guest has two skb - 4096 bytes and 1 bytes? In seqpacket there is
> > > > > special marker in header which shows where message ends, and how it works here?
> > > >
> > > > I think the main difference is that DGRAM is not connection-oriented, so
> > > > we don't have a stream and we can't split the packet into 2 (maybe we
> > > > could, but we have no guarantee that the second one for example will be
> > > > not discarded because there is no space).
> > > >
> > > > So I think it is acceptable as a restriction to keep it simple.
> > > >
> > > > My only doubt is, should we make the RX buffer size configurable,
> > > > instead of always using 4k?
> > > >
> > > I think that is a really good idea. What mechanism do you imagine?
> >
> > Some parameter in sysfs?
>
> I comment more on this below.
>
> > >
> > > For sendmsg() with buflen > VQ_BUF_SIZE, I think I'd like -ENOBUFS
> >
> > For the guest it should be easy since it allocates the buffers, but for
> > the host?
> >
> > Maybe we should add a field in the configuration space that reports some
> > sort of MTU.
> >
> > Something in addition to what Laura had proposed here:
> > https://markmail.org/message/ymhz7wllutdxji3e
>
> That sounds good to me.
>
> IIUC vhost exposes the limit via the configuration space, and the guest
> can configure the RX buffer size up to that limit via sysfs?
>
> > > returned even though it is uncharacteristic of Linux sockets.
> > > Alternatively, silently dropping is okay... but seems needlessly
> > > unhelpful.
> >
> > UDP takes advantage of IP fragmentation, right?
> > But what happens if a fragment is lost?
> >
> > We should try to behave in a similar way.
>
> AFAICT in UDP the sending socket will see EHOSTUNREACH on its error
> queue and the packet will be dropped.
>
> For more details:
> - the IP defragmenter will emit an ICMP_TIME_EXCEEDED from ip_expire()
>  if the fragment queue is not completed within time.
> - Upon seeing ICMP_TIME_EXCEEDED, the sending stack will then add
>  EHOSTUNREACH to the socket's error queue, as seen in __udp4_lib_err().
>
> Given some updated man pages I think enqueuing EHOSTUNREACH is okay for
> vsock too. This also reserves ENOBUFS/ENOMEM only for shortage on local
> buffers / mem.
>
> What do you think?

Yep, makes sense to me!

Stefano