From: Bill Burns <bburns@redhat.com>
Date: Fri, 15 Feb 2008 13:31:09 -0500
Subject: [xen] x86: revert to default PIT timer
Message-id: 47B5DA6D.70905@redhat.com
O-Subject: [RHEL5.2 PATCH] [Bug 428710] Kernel-xen hangs while trying to boot
Bugzilla: 428710

Resolves bz 428710

This fixes a regression where the Xen Hypervisor/kernel will
not boot on our Unisys es7000 as well as other systems with
lots of CPUs and large memory. The failure manifests itself
by dom0 reporting that time went backwards and getting hung
up during initialization and eventually crashing.

The problem can be worked around with boot args, but out of
the box the systems won't work. Adjusting the amount of memory
that the Hypervisor retains by either a dom0_mem arg or a xenheap
arg can alleviate the issue. Also specifying a different clock
source via a clocksource= arg will work around the problem.

The underlying problem is due to a change where the default
clock source was changed in the 3.1.2 Hypervisor. The new default
is the ACPI pmtimer. Previously the PIT timer was used for
these systems. The pmtimer code does not properly handle the
situation where at Hypervisor startup, when initializing
for a large number of CPUs and a lot of memory, interrupts
are held off for a long period. The result of this that
the speed parameters for CPU0 get incorrectly set. When the
dom0 kernel starts up and reads these parameters the symptom
occurs.

The fix simply removes the pmtimer from being automatically
selected over the PIT timer. The pmtimer can still be seleceted
via the clocksource=pmtimer Hypervisor arg.

This change is not upstream. There are various issues
around the pmtimer that are being talked about but no
real fix to the timer code appears to be coming forth.
This change logically backs out a small piece of the
3.1.2 Hypervisor rebase and reverts us to using the timers
we have used in the past.

Tested on the es7000 and desktop. Brew build:
http://brewweb.devel.redhat.com/brew/taskinfo?taskID=1171497

Please review and ack, thanks,
 Bill

Acked-by: Chris Lalancette <clalance@redhat.com>
Acked-by: Stephen Tweedie <sct@redhat.com>
Acked-by: Rik van Riel <riel@redhat.com>

diff --git a/arch/x86/time.c b/arch/x86/time.c
index 45edc44..92c5b8c 100644
--- a/arch/x86/time.c
+++ b/arch/x86/time.c
@@ -554,8 +554,7 @@ static void init_platform_timer(void)
 
     if ( (rc <= 0) &&
          !init_cyclone(pts) &&
-         !init_hpet(pts) &&
-         !init_pmtimer(pts) )
+         !init_hpet(pts) )
         init_pit(pts);
 
     plt_mask = (u32)~0u >> (32 - pts->counter_bits);