linux-kernel - Re: [x86-tip] RSDP changes converted i4790 box SMP -> UP

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Date:   Mon, 15 Jan 2018 16:08:50 +0100
From:   Mike Galbraith <mgalbraith@...e.de>
To:     Juergen Gross <jgross@...e.com>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...nel.org>
Subject: Re: [x86-tip] RSDP changes converted i4790 box SMP -> UP

On Mon, 2018-01-15 at 15:41 +0100, Juergen Gross wrote:
> On 12/01/18 09:01, Mike Galbraith wrote:
> > On Fri, 2018-01-12 at 07:40 +0100, Juergen Gross wrote:
> >> On 12/01/18 05:25, Mike Galbraith wrote:
> >>> Hi Juergen,
> >>>
> >>> Yesterday I wanted to test the RETPOLINE stuff in tip and tip-rt, but
> >>> discovered instead that my box had turned into a complete slug, not due
> >>> to incredible RETPOLINE overhead, rather because box had forgotten that
> >>> it had more than one CPU.  I was going to leave it for the weekend, but
> >>> firing up gitk over morning java, I noticed the commits below, and sure
> >>> enough, that's what broke my box.  Given other people's boxen work,
> >>> seems likely that the authors of the AMI BIOS in this box were a bit
> >>> more creative than usual.
> >>
> >> So I'm curious how this should be possible.
> > 
> > Box is absolutely certain that it is.
> > 
> >> Some questions:
> >>
> >> - which bootloader are you using?
> > 
> > grub2, box is bog standard opensuse-42.3
> 
> And this is the problem.
> 
> grub2 of opensuse 42.3 is _wrong_.
> 
> It copies the setup_header in the size it knows of (last fiels is
> handover_offset), fills in some stuff, and then copies back 1024 bytes.
> So grub2 of openSUSE 42.3 is _not_ ready to accept extensions of
> bootparams.
> 
> This is a major breakage of the protocol, I think.

Oh joy, userspace b0rkage.  Kernel b0rkage is a lot less annoying. 

Hohum, I have zero problem with carrying a revert until grub2 gets
fixed up, but lord knows how many others out there may bisect, and
bless your mailbox with the fruit of their labors :)

	-Mike