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

[Jamie Lokier <jamie@shareable.org>]

Amit Shah wrote:
> On (Wed) Jun 10 2009 [19:00:27], Jamie Lokier wrote:
> > Amit Shah wrote:
> > > > Plus, having a virtio serial device and a virtio console seems  
> > > > unfortunate to me.  Anyway we can unify the two?  Maybe make virtio  
> > > > serial support hvc with a feature flag or something instead of ttyS?
> > > 
> > > Yes, it's possible. I'll look into that once I reach a point where
> > > virtio-serial is ready to be merged. I'll also have to look at making
> > > sure existing guest kernels work with the new, unified device.
> > 
> > One of the most useful things about a serial port is that it's so easy
> > for basic OS components like debuggers and bootloaders and BIOSes to
> > use it, with very little code, and without having to do very complex
> > things like initialise and enumerate PCI buses etc., and without
> > interference by an OS so they are useful for things like kernel
> > debugging and reliable debugging output.
> > 
> > For this reason, virtually every real machine has a simple serial
> > port, even if it isn't exposed on an external connector.
> > 
> > Are virtio-serial/virtio-console very simple to drive from guest code,
> > without PCI enumeration, or do they require complex initialisation and
> > OS cooperation?
> 
> OK, great question. I'll post my Linux driver that'll answer your
> question precisely, but this is what a guest userspace app will have to
> do to use it:

Userspace doesn't matter so much - I'd assumed a simple char driver
carrying a pipe of bytes, no matter how the emulated hardware works.
But that's not relevant to the things I'm asking about, since they
run outside userspace, sometimes without a kernel.

> The driver itself hooks up with the virtio code which abstracts away all
> the pci stuff. It just has to manage the rings and the queues.

On the face of it, simply because virtio uses PCI, it looks a _lot_
more complicated than a serial port (UART) at a fixed I/O or MMIO
address, for things like boot loaders, boot ROMS, BIOS debugging,
kernel debuggers and minimal kernels.

I say on the face of it, because I don't know enough about how virtio
generally is set up in the guest.

Can you make virtio-serial start enabled with fixed, known I/O or MMIO
addresss, in a similar way to the way VGA cards and IDE ports are PCI
devices but can be used without PCI enumeration?

Can you make it pollable, so it's usable without having to route
interrupts, which can be quite complicated, or impossible when you're
debugging something that thinks it is control?

-- Jamie