linux-kernel - Re: reboot via bios on X86

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Date:	Mon, 09 Apr 2012 13:26:40 -0400
From:	Miles Fidelman <mfidelman@...tinghouse.net>
To:	Matthew Garrett <mjg@...hat.com>
CC:	linux-kernel@...r.kernel.org, tglx@...utronix.de, mingo@...hat.com,
	hpa@...or.com, x86@...nel.org, peter.chubb@...ta.com.au,
	michael.d.labriola@...il.com
Subject: Re: reboot via bios on X86_64?

Thanks for the quick response.

Matthew Garrett wrote:
> On Mon, Apr 09, 2012 at 11:39:32AM -0400, Miles Fidelman wrote:
>
>> 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?
> The bios reboot code requires you to reprogram the CPU to real mode. I
> don't think there's any fundamental reason you can't do that from 64-bit
> mode, but nobody's ever written that code.

That would explain it.  I guess there's a little bit of trickiness 
involved in setting things up before
switching to real mode.

>> 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).]
> Windows never uses the equivalent of the BIOS reboot method, so the
> assumption is that if you ever need on Linux to it's because there's a
> bug somewhere else. Does the platform reboot if you run the kernel on
> bare metal rather than under Xen?

No.  Tried all the combinations - w/ and w/o hypervisor, all the 
available kernel options.  Seems
like the only thing that will reboot this particular hardware/bios combo 
is via the bios.

Miles

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


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