reboot.c and X86-64 architectures

[mfidelman@meetinghouse.net (Miles Fidelman)]

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