Re: [Qemu-devel] [PATCH 5/8] virtio-serial-bus: Add support for buffering guest output, throttling guests

[Jamie Lokier <jamie@shareable.org>]

Amit Shah wrote:
> Guests send us one buffer at a time. Current guests send buffers sized
> 4K bytes. If guest userspace applications sent out > 4K bytes in one
> write() syscall, the write request actually sends out multiple buffers,
> each of 4K in size.
> 
> This usually isn't a problem but for some apps, like VNC, the entire
> data has to be sent in one go to make copy/paste work fine. So if an app
> on the guest sends out guest clipboard contents, it has to be sent to
> the vnc server in one go as the guest app sent it.
> 
> For this to be done, we need the guest to send us START and END markers
> for each write request so that we can find out complete buffers and send
> them off to ports.

That looks very dubious.  TCP/IP doesn't maintain write boundaries;
neither do pipes, unix domain sockets, pseudo-terminals, and almost
every other modern byte-oriented transport.

So how does VNC transmit the clipboard over TCP/IP to a VNC client,
without those boundaries, and why is it different with virtserialport?

-- Jamie