Re: [Qemu-devel] [PATCH 0/8] virtio-console: Move to qdev, multiple devices, generic ports

[Jamie Lokier <jamie@shareable.org>]

Anthony Liguori wrote:
> I think this is a pretty fundamental issue to work out since it 
> determines the very nature of the transport (stream vs. datagram).

For the record, I don't think there's anything _wrong_ with a datagram
transport.  It would be quite useful sometimes.  But if there is
datagram support, it should be optional, just like you can choose
between SOCK_STREAM and SOCK_DGRAM for AF_UNIX sockets.

Something else occurred to me with the cut buffer application:

What happens if the guest crashes, kexecs or whatever when it's half
way through sending a cut buffer?  A stream protocol will not have a
nice way to recover from that unless there is an additional "out of
band" way to say "I'm starting again".  Does virtio-serial have an
"I'm starting again" which is passed to the host side application?

> Because you have to put a max buffer size on the transport, I think 
> buffering is a really flawed approach provably equivalent to just 
> increasing the message size within the transport.  In general, the later 
> is a better approach because then the guest is using it's memory vs. 
> using host memory.

I agree, using guest memory for the buffer also means there doesn't
have to be an arbitrary limit on the buffer size, or a time limit.  It
can just wait there until it's consumed or the guest decides to restart.

-- Jamie