Re: [Qemu-devel] [PATCH 3/6] virtio-console: Add a virtio-console bus, support for multiple ports

[Gerd Hoffmann <kraxel@redhat.com>]

> BTW the kernel too doesn't support multiple devices so far.

Needs fixing too ;)

>> I'd suggest to look what do do when another user which wants buffering
>> comes along.  Might be the requirements are different, so it wouldn't
>> work anyway.
>>
>> Also when you pass around pointers to VirtConPortBuffer the port drivers
>> can implement buffering very easily.
>
> It will mostly be duplicated. Right now, I pass on an entire message as
> was passed on by the kernel in one write() request. If I start passing
> around buffers, it'll be incomplete messages (eg, an 8K write request in
> the kernel will be split in two buffers of 4K each -- or three buffers
> with the last one containing a few bytes). I don't think I want the ports
> to worry about that. The ports should just get the entire message.

Sure.  I think there is a misunderstanding here.  I meant only the 
"buffer messages when unconnected" thing.  Reassembling the messages in 
the core and forward only complete messages to the ports is fine.

> So I don't think I want to expose the buffers to individual devices.

My idea was to pass the (reassembled) message to the port drivers in a 
form they can just stick into a list without memcpy()ing the message in 
case they want to keep it.  Maybe to buffer it because the chardev is 
unconnected.  Maybe just keep the most recent message because it 
contains some kind of guest status.

VirtConPortBuffer looked like it could do the job.  Well, not perfectly, 
it has extra status which is needed only by the core for reassembling 
messages.

>> They are handled differently though.  I think console will not do any
>> buffering at all, whereas serial-port provides the option to do
>> buffering.
>
> But the buffering is independent of the char drivers.

See above ;)

> Basically I've now dropped the old -virtioconsole argument.
>
> So one has to do:
>
> -device virtio-console-pci -device virtioconsole ...
>
> to start a console.

If you do '-device virtio-console-pci -device virtio-port' (i.e. no 
console) and boot a old guest kernel which expects a (single) console 
being there, what will happen?

cheers,
   Gerd