+++ This bug was initially created as a clone of Bug #201298 +++

The Intel test machines of model number D3C51SYBANCNRB fail to reboot under Xen
if VMX is enabled in the BIOS.  The reboot code of Xen is identical to that of
Linux, in particular, it attempts a reboot via the keyboard reset line, and if
that fails, it tries to create a triple fault by zeroing the IDT.

This works perfectly if VMX is diabled in the BIOS.  This also works if
start_vmx is not called from Xen.  In fact, the reboot still works if start_vmx
is followed immediately by a call to stop_vmx.

Booting Xen with reboot=b (which uses a different reboot strategy where it jumps
into a BIOS address in real mode) causes it to reboot correctly even with VMX
enabled.

What I'd like to know from Intel is

1) What is the reason that the aformentioned reboot strategies (keyboard +
triple fault, a standard reboot strategy for many years in Linux) fails when VMX
was enabled in the past?
2) If you think this reboot strategy is flawed, what reboot strategy do you
recommend that can work across the full range of x86 hardware that Linux/Xen
supports?

-- Additional comment from yunfeng.zhao on 2006-08-04 11:54 EST --
We also have the same problem.
Native RHEL4/SLES10 could be rebooted on Conroe without any problems, but xen0 
could not be rebooted.
To reboot xen0 on Conroe we have to press reset button manually.
We thought it's a hardware fault of Conroe.

I will recheck this issue, and see if it's a bug xen vt.

thanks
Yunfeng

-- Additional comment from bstein on 2006-08-14 11:11 EST --
Yunfeng - any update?

-- Additional comment from sct on 2006-08-21 10:23 EST --
Also fails for me on the same product code.  Attaching successful and failing
boot logs in case they are useful.

-- Additional comment from sct on 2006-08-21 10:26 EST --
Created an attachment (id=134562)
Boot logs from kernel-2.6.17-1.2573.fc6PAE

Reboot/poweroff work correctly on this kernel.

-- Additional comment from sct on 2006-08-21 10:27 EST --
Created an attachment (id=134563)
Boot logs from kernel-2.6.17-1.2573.fc6xen

Reboot/poweroff always hang on this kernel.

-- Additional comment from herbert.xu on 2006-09-03 20:31 EST --
Thanks for the dmesgs Stephen.  At this point I'm only interested in poweroff
since your reboot issue can be explained by the fact that Xen enables VMX while
baremetal does not.

I can see two differences between your setup and mine.  Firstly your ACPI BIOS
is different to mine, and secondly I need to test using the same kernel as
you're to see if that could make my baremetal poweroff consistently.

I've observed an interesting phenomon with poweroff on my machine.  It seems to
work if I leave it either on or off for an extended period of time.  However, it
only works once.  That is, if I power it back on after a successful poweroff and
immediately try to halt, it fails to power off.

So could you do an experiment for me? See if you could do three or four
successive poweroffs (on baremetal of course) and let me know whether they all
succeed.

-- Additional comment from sct on 2006-09-04 06:25 EST --
Using the 1.2600 PAE kernel, bare metal poweroff failed on the first attempt.  

Strange, as it used to work reliably; but then again I haven't tried it
recently, as all my recent work has been using a -xen kernel on that box, and
I've just got used to having to poweroff manually with that kernel.