linux-kernel - Re: reboot via bios on X86

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Message-ID: <20120409155155.GA22974@srcf.ucam.org>
Date:	Mon, 9 Apr 2012 16:51:55 +0100
From:	Matthew Garrett <mjg@...hat.com>
To:	Miles Fidelman <mfidelman@...tinghouse.net>
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?

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.

> 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?

-- 
Matthew Garrett | mjg59@...f.ucam.org
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