linux-kernel - Re: [BUG] x86: reboot doesn't reboot

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Message-ID: <20140404174437.GC25173@gmail.com>
Date:	Fri, 4 Apr 2014 19:44:37 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	"H. Peter Anvin" <hpa@...or.com>
Cc:	Matthew Garrett <mjg59@...f.ucam.org>,
	"Li, Aubrey" <aubrey.li@...ux.intel.com>,
	Steven Rostedt <rostedt@...dmis.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [BUG] x86: reboot doesn't reboot

* H. Peter Anvin <hpa@...or.com> wrote:

> The comment header is bogus... it describes what we do, not what 
> Windows does.

That comment certainly pretends to describe Windows behavior and then 
it goes to outline the differences in Linux behavior:

/*
 * Windows compatible x86 hardware expects the following on reboot:
 *
 * 1) If the FADT has the ACPI reboot register flag set, try it
 * 2) If still alive, write to the keyboard controller
 * 3) If still alive, write to the ACPI reboot register again
 * 4) If still alive, write to the keyboard controller again
 * 5) If still alive, call the EFI runtime service to reboot
 * 6) If still alive, write to the PCI IO port 0xCF9 to reboot
 * 7) If still alive, inform BIOS to do a proper reboot
 *
 * If the machine is still alive at this stage, it gives up. We default to
 * following the same pattern, except that if we're still alive after (7) we'll
 * try to force a triple fault and then cycle between hitting the keyboard
 * controller and doing that
 */

(The 'we default' means 'Linux defaults'.)

So that comment should be fixed to describe the hardware ABI (Windows 
behavior), to the extent it is known - and then outline where and why 
Linux deviates from that. (and preferably it should match Windows 
behavior and only _add_ to that behavior at the end of the reboot 
sequence.)

Before any other change is done beyond the revert of this latest 
broken commit.

Thanks,

	Ingo
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