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

[Stefano Garzarella]

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?

> 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

> 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.

> FYI, this patch is broken for h2g because it requeues partially sent
> skbs, so probably doesn't need much code review until we decided on the
> policy.

Got it.

Thanks,
Stefano