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Re: identifying module types

From: Hollis Blanchard
Subject: Re: identifying module types
Date: Fri, 08 Dec 2006 18:02:31 -0600

On Fri, 2006-10-27 at 06:09 +0200, Tristan Gingold wrote:
> BTW, why not adding a type field for module tag.  The type (which should be
> an UUID IMHO) should indicate the type of the module.
> One usage could be for Xen.  On Xen you can load 3 modules: the linux kernel,
> the linux ramdisk and an ACM configuration.  Xen relies on order and on some
> magic checks to find the module type.
> The command syntax could be:
> module [-type TYPE] file [cmdline]

As I'm implementing the Xen side of this, I can now see the need.

Xen uses a handful of modules:
- xen kernel
- dom0 kernel
- dom0 initrd
- security policy (binary blob)
- possibly others

On the consumer side of multiboot (in this case Xen), we need to loop
over the tags, and when we find a module tag, how do we know which it
is? The Multiboot2 spec tells us "The order of modules is not
guaranteed." (Why not?)

If we can't rely on the order, then we have no reliable way to
distinguish the type of module we're looking at, so a type field would
be extremely useful. For example:
        multiboot (hd1,1)/xen
        module -t xenhv-dom0 (hd1,1)/vmlinux
        module -t xenhv-dom0-initrd (hd1,1)/initrd
        multiboot (hd0,0)/boot/gnumach.gz root=device:hd2s1
        module -t hurd-something (hd0,0)/lib/

One option is a fixed-length encoded field, say 32 bytes wide. To avoid
namespace collisions, we could require that projects prefix types with
their project name, which must be at least 4 bytes.

Another alternative would be a NULL-terminated string, which would
appear in memory just before the NULL-terminated command line, e.g.
        x e n \0 c o n s o l e = c o m 2 \0
This is more flexible, but slightly more awkward on the consumer side:
        type = module_tag->text;
        cmdline = strchr(module_tag->text, '\0') + 1;


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