reboot.c and X86-64 architectures

[Srivatsa S. Bhat]

On 04/04/2012 07:18 PM, Miles Fidelman wrote:

> Jim Cromie wrote:
>> On Tue, Apr 3, 2012 at 9:22 PM, Miles Fidelman
>> <mfidelman at 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.
> 


I suggest you post this issue on lkml, with CC to the authors/maintainers
of the code, which would be the following list, as per get_maintainer.pl:

Thomas Gleixner <tglx at linutronix.de>
Ingo Molnar <mingo at redhat.com>
"H. Peter Anvin" <hpa at zytor.com>
x86 at kernel.org
Peter Chubb <peter.chubb at nicta.com.au>
Michael D Labriola <michael.d.labriola at gmail.com>
Matthew Garrett <mjg at redhat.com>
linux-kernel at vger.kernel.org

 
Regards,
Srivatsa S. Bhat