[SeaBIOS] [PATCH] Use dedicated HVM debug io port for Xen

[Kevin O'Connor]

On Thu, Jan 05, 2012 at 12:26:31AM +0000, Julian Pidancet wrote:
> On Thu, Jan 5, 2012 at 12:05 AM, Kevin O'Connor <kevin at koconnor.net> wrote:
> > On Wed, Jan 04, 2012 at 05:19:59PM +0000, Julian Pidancet wrote:
> >> On Sat, Nov 5, 2011 at 2:43 AM, Julian Pidancet
> >> <julian.pidancet at gmail.com> wrote:
> >> > When running on Xen, use Xen debug io port as debug output instead
> >> > of Bochs one.
> >
> > I'm leery of having multiple "magic" debug ports.  Is it possible for
> > Xen to pick up port 0x0402 (or use the qemu style -device
> > isa-debugcon,iobase=0x402 command line option)?
> >
> 
> Well, I initially thought that 0x402 was a bochs specific debug port.
> Now I'm not too sure about this any longuer, because I just found in
> the bochs documentation that 0xe9 can be used as well:
> http://bochs.sourceforge.net/doc/docbook/user/bochsrc.html
> (port_e9_hack)
> 
> Can you enlighten me as to the origin of the 0x402 port ?

As far as I know, it's a Bochs BIOS thing (see rombios.h).  Bochs
seems to have lots of debug ports (eg, 0x400-0x403) - I'm not sure
why.  Qemu also has code for port 0x402 if DEBUG_BIOS is enabled in
hw/pc.c - though it's much easier to use the isa-debugcon command line
option.

Also note that the DBUG() method in SeaBIOS' src/acpi-dsdt.dsl is also
setup to use port 0x402 right now.

> I really doubt picking up a new debug port in xen would be acceptable
> because 0xe9 has been around for a long time and it would certainly
> break legacy code.

Is the issue that writes to 0x402 are causing an issue on Xen, or just
that you don't see the debug output?

What's the origin of 0xe9 - is it a "standard" for debug output?  What
does Xen do today on writes to 0xe9?

-Kevin