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Re: [Xen-devel] Serial console problem with Xen 3.0

On Fri, 2006-04-07 at 10:44 -0400, Don Zickus wrote:
> > 
> > Me too. Adding "sync_console" at least repaired the console *output*. I 
> > still have the problem that keyboard input does not reach linux, as it 
> > seems. Here is my config:
> > 
> Adding "pnpacpi=off" to the command line is our quick and dirty hack for
> now.  Don't know the root cause yet, but at least it will get you going.
> Cheers,
> Don

I had a similar problem during some of my development. Whenever I booted
Xen with a serial console, the serial output would quit shortly after
dom0 booted and then the system would "hang" at random places. I tracked
it down to the pnpacpi code as well and I think I know (now) what the
problem is. ACPI provides a means to disable the devices (the _DIS
method). Serial ports often appear as ACPI devices and when pnpacpi
discovers that no driver is using a device, it disables it (by calling
the _DIS method). Xen thinks it has exclusive access to the serial port
(I believe it blocks dom0 from accessing the related i/o ports), but
dom0 has an out-of-band way (through ACPI) of messing with the serial
port (_DIS isn't the only method in ACPI that can be used to change the
properties of the serial port). I believe my system was subsequently
hanging because the buffer to the serial port got full and it was
waiting for it to empty (although I didn't ever verify this part).

I've not yet found a good workaround except to disable pnpacpi. It would
be nice to have pnpacpi (or some component) detect that Xen is using the
serial port and leave it untouched.

I think this explains the behavior in bug 229 (and possibly 579 as


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