Re: [Xen-devel] Uniform commands for booting xen

From: "Jan Beulich" <JBeulich@suse.com>
To: "Ian Campbell" <ian.campbell@citrix.com>,<phcoder@gmail.com>
Cc: The development of GRUB 2 <grub-devel@gnu.org>,
	"xen-devel@lists.xen.org" <xen-devel@lists.xen.org>
Subject: Re: [Xen-devel] Uniform commands for booting xen
Date: Fri, 13 Nov 2015 00:48:40 -0700	[thread overview]
Message-ID: <5645A3E802000078000B495A@prv-mh.provo.novell.com> (raw)
In-Reply-To: <1447348140.18450.92.camel@citrix.com>

>>> On 12.11.15 at 18:09, <ian.campbell@citrix.com> wrote:
> On Thu, 2015-11-12 at 08:44 -0700, Jan Beulich wrote:
>> > > > On 12.11.15 at 14:41, <phcoder@gmail.com> wrote:
>> > Hello, all. I'd like to have set of commands that would boot xen on all
>> > platforms. I thought of following set:
>> > 
>> > xen_hypervisor FILE XEN_OPTIONS
>> > xen_kernel FILE KERNEL_OPTIONS
>> > xen_initrd INITRD INITRD INITRD
>> > all initrds are concatenated.
>> > xen_xsm ???
>> 
>> xen_ucode (and we might add more going forward). I don't see
>> why the multiboot mechanism (kernel plus any number of modules)
>> can't be used, without adding any Xen-specific directives.
> 
> You likely aren't aware that on ARM Xen doesn't boot via multiboot, but via
> a protocol which involves passing modules in an fdt[0].
> 
> I had originally hoped that this would use the same command names in the
> grub cfg, such that things would just work, however the grub maintainers
> didn't like that (and I appreciate why).
> 
> Hence on grub/ARM we already have xen_{hypervisor,kernel,initrd,...}.
> 
> The question then is what grub-mkconfig (or more precisely
> /etc/grub.d/20_linux_xen) ought to emit so that things just work on all
> architectures.
> 
> The author of the grub/ARM/Xen patches initially made it generate the xen_*
> namas for arm and the multiboot names for x86, here is Vladimir's feedback
> on that: http://lists.gnu.org/archive/html/grub-devel/2015-10/msg00133.html 
> 
> Which I think gets us to approximately today and Vladimir's question.

Now that makes the situation really ugly (and supports my
reservations regarding grub2 as a uniform solution for everything).
How do you express modules other than kernel+initrd in that
scheme, without grub needing to be aware of any new addition we
may find necessary going forward?

I think any architecture following a well defined, cross-arch
protocol (like multiboot) should not require any special xen_*
directives. If ARM64 needs Xen to be treated specially, special
directives are maybe warranted for this particular case, but I don't
see why all architectures supporting Xen should then automatically
have to use those too. But yes, it's not my call decide this...

Jan