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Re: [libvirt] [PATCH 1/2] conf: add debugcon-isa chardev guest interface



On Mon, Jan 28, 2019 at 11:26:57AM +0000, Daniel P. Berrangé wrote:
> On Mon, Jan 28, 2019 at 12:19:36PM +0100, Andrea Bolognani wrote:
> > On Mon, 2019-01-28 at 10:36 +0000, Daniel P. Berrangé wrote:
> > > On Mon, Jan 28, 2019 at 11:24:38AM +0100, Andrea Bolognani wrote:
> > > > Should this be a <channel> at all?
> > > > 
> > > > We generally use <serial> for native emulated devices such as
> > > > isa-serial, which isa-debugcon seems very closely related to...
> > > 
> > > <channel> is aiming to expose application specific guest data channels.
> > > 
> > > <serial> is aiming to expose general purpose guest data channels.
> > > 
> > > To me isa-debugcon is an application specific data channels, so
> > > <channel> feels a better fit.
> > 
> > Maybe I have misunderstood what isa-debugcon is, but it looks more
> > of a general purpose channel in the vein of isa-serial rather than
> > something application specific like the qemu-guest-agent or SPICE
> > channels for example.
> 
> My understanding is that isa-debugcon is a special device just
> for usage by SeaBIOS to emit debug messages.

Well, it's a primitive, output only device with a single, configurable
I/O port.  Every byte written to the io port is send to the chardev.

It's used in practice for firmware debug output.  seabios and OVMF
support this (coreboot too IIRC).  And even though the port is
configurable pretty much everybody agreed on using iobase=0x402.

I'd classify this as serial device:
    <serial><target type='debugcon'/></serial>

And, yes, you surely don't want use that as <console>.

cheers,
  Gerd


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