Serial console hangs with Linux 2.6.20 HVM guest

[Anders Kaseorg <andersk@MIT.EDU>]

I am seeing a problem with the Xen emulated serial console.  When
running a Linux 2.6.20 HVM guest that has CONFIG_HOTPLUG_CPU=n, the
guest blocks on output to the console until it receives input keypresses
from `xm console`.  This prevents the guest from booting up without
banging on some keys, and makes interactive use of the console
difficult.

By bisecting Linux kernel commits, I found that the bug goes away in
commit 40b36daad0ac704e6d5c1b75789f371ef5b053c1 (v2.6.21-rc1~261), which
is a workaround for buggy UARTs on certain HP machines.

http://git.kernel.org/?p=linux/kernel/git/torvalds/linux-2.6.git;a=commit;h=40b36daad0ac704e6d5c1b75789f371ef5b053c1

“The patch below works around a minor bug found in the UART of the
remote management card used in many HP ia64 and parisc servers (aka the
Diva UARTs).  The problem is that the UART does not reassert the THRE
interrupt if it has been previously cleared and the IIR THRI bit is
re-enabled.  This can produce a very annoying failure mode when used as
a serial console, allowing a boot/reboot to hang indefinitely until an
RX interrupt kicks it into working again (ie. an unattended reboot could
stall).”

That matches my symptoms exactly, which suggests that the Xen UART
probably has a similar bug.

I’ve seen this in Xen 3.2.1 and 3.3.1.  (My host is running Debian Lenny
amd64, with the Xen dom0 kernel 2.6.24-23-xen from Ubuntu Hardy, on a
server with two quad-core Xeons.)

Anders