QUESTION: reboot.c and X86-64 architectures

[Miles Fidelman <mfidelman@meetinghouse.net>]

Hello Folks,

I hope this is an appropriate question here, I've exhausted other resources:

I've recently migrated from running a 32bit kernel to a 64bit one 
(specifically Debian Lenny 32bit environment to 2.6.32-5-xen-amd64 on 
top of xen-4.0-amd64 hypervisor build).

This is a somewhat older, and apparently quirky, hardware box.  I've 
found that the only way to reboot it, short of power cycling, is jumping 
through the bios - using the "reboot=bios" kernel option at boot time 
works just fine for an X86_32 kernel.  But... this doesn't work with the 
64bit kernel.

Pouring through both the documentation and the code for linux 
<http://lxr.linux.no/linux+*/>/arch 
<http://lxr.linux.no/linux+*/arch>/x86 
<http://lxr.linux.no/linux+*/arch/x86>/kernel 
<http://lxr.linux.no/linux+*/arch/x86/kernel>/reboot.c 
<http://lxr.linux.no/linux+*/arch/x86/kernel/reboot.c> yields the very 
specific admonition "bios Reboot by jumping through the BIOS (only for 
X86_32)", and the code seems to support this (though I haven't quite 
waded through all the paths that might execute machine_real_restart).

None of the other available command line options seem to work with this 
hardware, which leads to two questions:

1. What's the logic behind this?  Why not enable a bios reboot for 64bit 
kernels?

2. Anybody know a workaround, short of patching and compiling a custom 
kernel? (Note: None of the available command line options seem to work on
this particular box/bios combination, and kexec-reboot is not available 
for this combination of kernel and hypervisor).

Thank you very much,

Miles Fidelman

-- 
In theory, there is no difference between theory and practice.
In practice, there is.   .... Yogi Berra