reboot.c and X86-64 architectures

[mfidelman@meetinghouse.net (Miles Fidelman)]

Hello Folks,

Perhaps someone here can help me understand the behavior of the kernel reboot code.

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
arch/x86/kernel/reboot.c yields the very specific admonition (in comments
describing command-line kernel options), that reboot=bios (Reboot by jumping
through the BIOS) only applies to X86_32 - and the case statements in the
code seem to align with the comment.

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

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

2. Anybody know a workaround, short of patching and compiling a custom
kernel? Are there other paths through the reboot code that can invoke a bios
reboot?  (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