linux-kernel - Re: [Xen-devel] [PATCH] xen: remove unused Kconfig parameter

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Date:	Tue, 9 Jul 2013 23:19:34 -0700
From:	Matt Wilson <msw@...zon.com>
To:	Sander Eikelenboom <linux@...elenboom.it>
CC:	Borislav Petkov <bp@...en8.de>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>,
	<jeremy@...p.org>, <xen-devel@...ts.xensource.com>,
	Jan Beulich <JBeulich@...e.com>,
	Michael Opdenacker <michael.opdenacker@...e-electrons.com>,
	<x86@...nel.org>, Paul Bolle <pebolle@...cali.nl>,
	<virtualization@...ts.linux-foundation.org>, <mingo@...hat.com>,
	"H. Peter Anvin" <hpa@...or.com>, <tglx@...utronix.de>,
	<linux-kernel@...r.kernel.org>
Subject: Re: [Xen-devel] [PATCH] xen: remove unused Kconfig parameter

On Wed, Jul 10, 2013 at 12:34:58AM +0200, Sander Eikelenboom wrote:
> 
> Tuesday, July 9, 2013, 5:05:54 PM, you wrote:
> 
> > On Tue, Jul 09, 2013 at 10:48:40AM -0400, Konrad Rzeszutek Wilk wrote:
> >> Then that should be discussed on grub2 to remove said check and modify
> >> the code so that it can properly work without regression.
> 
> > Actually, the kernel patch removing that symbol should be applied so
> > that grub2 breaks faster. One can't possibly rely on kernel internals
> > for anything, as it is insanely insane (yep, the tautology is on purpose
> > :-)).
>
> How insanely insane is it to be able to determine whether a certain
> compiled kernel binary supports a certain function ?
> 
> Grub does this in it's update script to prevent adding a xen +
> kernel combination that has no chance of booting when dom0 support
> has not been configured in the kernel.  That doesn't seem to be a
> unreasonable thought.
> 
> Grepping the accompanied config file in /boot for the xen dom0
> Kconfig parameter seems the best possible effort grub can do at the
> moment.

I think this can be improved, even with the situation today.

> Especially since the Kconfig parameter naming doesn't change that
> often.
> 
> If you know a better way for grub to determine if a certain function
> for a kernel binary is supported then please elaborate ..

Certainly. Parse the ELF notes that are present in a dom0-capable
Linux kernel binary itself.

$ readelf -n vmlinux

Notes at offset 0x0069be88 with length 0x0000017c:
  Owner		Data size	Description
  Xen		0x00000006	Unknown note type: (0x00000006)
  Xen		0x00000004	Unknown note type: (0x00000007)
  Xen		0x00000008	Unknown note type: (0x00000005)
  Xen		0x00000008	Unknown note type: (0x00000003)
  Xen		0x00000008	NT_VERSION (version)
  Xen		0x00000008	NT_ARCH (architecture)
  Xen		0x0000002a	Unknown note type: (0x0000000a)
  Xen		0x00000004	Unknown note type: (0x00000009)
  Xen		0x00000008	Unknown note type: (0x00000008)
  Xen		0x00000010	Unknown note type: (0x0000000d)
  Xen		0x00000004	Unknown note type: (0x0000000e)
  Xen		0x00000008	Unknown note type: (0x0000000c)
  Xen		0x00000008	Unknown note type: (0x00000004)
  GNU		0x00000014	NT_GNU_BUILD_ID (unique build ID bitstring)

See arch/x86/xen/xen-head.S.

There's a new note type (XEN_ELFNOTE_SUPPORTED_FEATURES) that we can
use to make dom0 support explicit.

http://xenbits.xen.org/gitweb/?p=xen.git;a=blob;f=xen/arch/x86/domain_build.c;hb=HEAD#l415

--msw
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