From: Bill Burns <bburns@redhat.com>
Date: Wed, 12 Mar 2008 09:10:08 -0400
Subject: [xen] HV inside a FV guest, crashes the host
Message-id: 47D7D630.6050409@redhat.com
O-Subject: [RHEL 5.2 PATCH] Fix Xen hypervisor inside a fullyvirt guest, crashes the host
Bugzilla: 436351

Fixes bz 436351

The bz is a regression complaint that the Xen Hypervisor and
dom0 coud not be run in a fully virtualized guest. This previously
worked. The result was that the host Hypervisor would crash
with a divide by zero error.

The fix was found upstream. It prevents doing missed
tick cacluations on one shot timers. The fix is very
simple, but the upstream patch situation is not.
The fix exists in patches both on the Xen unstable and
3.1.x branches. Idelly we'd pull the 3.1.x changeset and be done
with it, since our Hypervisor is 3.1.2. But the changeset
as applied to the 3.1 stream gloms together other, somewhat
related items that we do not want. Indeed even the unstable
changeset has two sections. The first section fixes this bug and
the second introduces yet another bug where a timer going
off is lost (which results in a hang, and not a crash). While
there are further changesets to fix the new problem the tack
taken here is to only take the "fix" and not the "bug" from the
original changeset. This minimizes our risk  by not taking
in extra code.

The upstream changeset this patch is based on is:
  http://xenbits.xensource.com/staging/xen-unstable.hg?rev=15943

For reference the 3.1 upstream changeset is:
  http://xenbits.xensource.com/staging/xen-3.1-testing.hg?rev=15550
with further cleanup fixes in
  http://xenbits.xensource.com/staging/xen-3.1-testing.hg?rev=15561

Brewbuild:
http://brewweb.devel.redhat.com/brew/taskinfo?taskID=1208011

Built locally and under brew, tested on x86_64 system.
Problem was reproduced and fixed. New images ran both as
the host and in the FV guest.

Please review and ack.

Thanks,
 Bill

# patch derived from Xen unstable:
#changeset:   15550:03551b644e35
#user:        Keir Fraser <keir.fraser@citrix.com>
#date:        Wed Dec 12 14:41:39 2007 +0000
#summary:     [HVM] Don't count "missed ticks" on one-shot timers.
# Note that only the first part of the upstrem changeset, as the
# second part introduces another bug.
#

Acked-by: "Stephen C. Tweedie" <sct@redhat.com>
Acked-by: Don Dutile <ddutile@redhat.com>
Acked-by: Chris Lalancette <clalance@redhat.com>

diff --git a/arch/x86/hvm/vpt.c b/arch/x86/hvm/vpt.c
index 5ea7303..4762ef6 100644
--- a/arch/x86/hvm/vpt.c
+++ b/arch/x86/hvm/vpt.c
@@ -46,6 +46,9 @@ static void missed_ticks(struct periodic_time *pt)
 {
     s_time_t missed_ticks;
 
+    if ( pt->one_shot )
+        return;
+
     missed_ticks = NOW() - pt->scheduled;
     if ( missed_ticks <= 0 )
         return;