linux-kernel - reboot via bios on X86

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Date:	Mon, 09 Apr 2012 11:39:32 -0400
From:	Miles Fidelman <mfidelman@...tinghouse.net>
To:	linux-kernel@...r.kernel.org
CC:	tglx@...utronix.de, mingo@...hat.com, hpa@...or.com,
	x86@...nel.org, peter.chubb@...ta.com.au,
	michael.d.labriola@...il.com, mjg@...hat.com
Subject: reboot via bios on X86_64?

[cc'd to maintainers of reboot.c]

Re. the 2.6.32.5 kernel (currently used in Debian stable), and this 
seems to apply to
later ones as well:

The reboot code in arch/x86/kernel/reboot.c provides for rebooting 
through the bios
by either a kernel option of "reboot=bios" or as automatically invoked 
by quite
a few of the quirks case statements.

But... reboot=bios is  enabled only when configured for 32-bit mode - 
which just bit me
when I moved from a 32bit kernel to a 64bit one on an old P4 box, which 
hangs on
reboot for all reboot options except reboot=bios when running in 32 bit 
mode.

Which leads to two questions:

1. What's the logic behind this?  Why not enable a bios reboot for 64bit
kernels?  Is there any reason why the machine_real_restart code wouldn't
work just as well in 64bit mode?  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: someone suggested trying a kexec-reboot, but that's also
not supported on my configuration (running over a Xen hypervisor).]

Thank you very much,

Miles Fidelman

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

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