Jim Cromie wrote:
On Tue, Apr 3, 2012 at 9:22 PM, Miles Fidelman <mfidelman@meetinghouse.net> wrote:
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).
perhaps you should try a non-hypervisor kernel. I have no experience with one, but it could be the issue. Virtualization developers tend to have big fast modern boxes, and as you say, yours isnt so modern.
It's not related to the hypervisor - I've tried. It's related specifically to the quirks of this particular machine/bios combination. At this point, it's more an itch I'm trying to scratch, this is a sandbox machine under my desk, not one of our production boxes - it's easy enough to shutdown then hit the power button. But... it really raises the question: Why does the reboot.c code only support reboot=bios (switch to real mode, set up a few memory locations, jump into the bios) for X86_32, and not for X86_64. The hardware and bios don't change when loading a 64-bit kernel, or am I missing something? SOMEBODY wrote that code. SOMEBODY should know the logic. But... I've drawn a blank so far. Miles Fidelman -- In theory, there is no difference between theory and practice. In practice, there is. .... Yogi Berra