linux-kernel - Re: reboot via bios on X86

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Date:	Mon, 09 Apr 2012 10:53:07 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Miles Fidelman <mfidelman@...tinghouse.net>
CC:	Matthew Garrett <mjg@...hat.com>, linux-kernel@...r.kernel.org,
	tglx@...utronix.de, mingo@...hat.com, x86@...nel.org,
	peter.chubb@...ta.com.au, michael.d.labriola@...il.com
Subject: Re: reboot via bios on X86_64?

On 04/09/2012 10:26 AM, Miles Fidelman wrote:
> 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.
> 

Actually this code already exists; the difference is that we have to
return to 32-bit mode first.

	-hpa
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