Re: [Qemu-devel] Re: [PATCH] virtio-serial: PCI device for simple host <-> guest communication

["Richard W.M. Jones" <rjones@redhat.com>]

On Tue, Jun 09, 2009 at 07:15:25PM +0200, Jan Kiszka wrote:
> Amit Shah wrote:
> > This interface presents a char device from which bits can be
> > sent and read.
> > 
> > Sample uses for such a device can be obtaining info from the
> > guest like the file systems used, apps installed, etc. for
> > offline usage and logged-in users, clipboard copy-paste, etc.
> > for online usage.
> 
> Out of curiosity: What is the advantage over instantiating a multiport
> 16550A PCI card? Will those channels be so performance critical?

16550A has a couple of problems: The fixed sized FIFO (16 bytes) can
cause problems if you send data too quickly.  (I've seen this problem
when sending too much data from host to the guest, but I can't find
the bug report right now ...).  Secondly UARTs are complicated to
emulate compared to a clean virtio device.  Would we be happy with the
address space taken up and amount of emulation going on once we had,
say, 16 emulated ports?

On the other hand, multiport cards don't require special drivers in
the guest ...

Rich.

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