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

[Bobby Eshleman <bobbyeshleman@gmail.com>]

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?

For sendmsg() with buflen > VQ_BUF_SIZE, I think I'd like -ENOBUFS
returned even though it is uncharacteristic of Linux sockets.
Alternatively, silently dropping is okay... but seems needlessly
unhelpful.

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.

Best,
Bobby