Re: [PATCHv2 0/4] x86: 5-level related changes into decompression code

[H. Peter Anvin]

On 11/29/17 09:48, Borislav Petkov wrote:
> On Wed, Nov 29, 2017 at 08:08:31PM +0300, Kirill A. Shutemov wrote:
>> We're really early in the boot -- startup_64 in decompression code -- and
>> I don't know a way print a message there. Is there a way?
>>
>> no_longmode handled by just hanging the machine. Is it enough for no_la57
>> case too?
> 
> Patch pls.
> 

I don't think there is any way to get a message out here.  It's too late
to use the firmware, and too early to use anything native.

no_longmode in startup_64 is an oxymoron -- it simply can't happen,
although of course we can enter at the 32-bit entry point with that problem.

We can hang the machine, or we can triple-fault it in the hope of
triggering a reset, and that way if the bootloader has been configured
with a backup kernel there is a hope of recovery.

Triple-faulting is trivial:

	push $0
	push $0
	lidt (%rsp)		/* %esp for 32-bit mode */
	ud2
	/* WTF? */
1:	hlt
	jmp 1b

This will either hang the machine or reboot it, depending on if the
reboot-on-triple-fault logic in the chipset actually works.

	-hpa