docs: Introduce specification for x86 pv bootloader chainloading paths/formats.

In order to support pvgrub (or other bootloader) from dom0 chainloading a
pvgrub (or other) from within the domU filesystem we need a standard for where
the stage 1 bootloader should look and what it should expect to find there.

Add a document along those lines.

This is currently x86-specific since it is expected that other new architecures
(including ARM) will either not need to implement this due to the existence of
more generic specifications for that arch (e.g. the ARM VM spec) or will update
this document in due course.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Acked-by: Ian Jackson <Ian.Jackson@eu.citrix.com>
Cc: Colin Watson <cjwatson@debian.org>
Cc: 759018@bugs.debian.org

diff --git a/docs/misc/x86-xenpv-bootloader.markdown b/docs/misc/x86-xenpv-bootloader.markdown
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+# Xen x86 PV Bootloader Protocol
+
+## Introduction
+
+One method for booting an x86 Xen PV guest is to use a PV bootloader,
+that is, a bootloader which is itself a PV kernel but which behaves as
+a bootloader (examples include the pvgrub-legacy and grub2 targeting
+Xen).
+
+In many cases the user wishes to manage this PV bootloader from within
+the guest, and therefore wishes to chainload something from the guest
+filesystem, most likely via a stage 1 PV bootloader provided by dom0.
+
+The purpose of this document is to define the paths within the guest
+filesystem where a stage 1 bootloader should look for the in-guest PV
+bootloader to load and the protocol/format expected from the
+to-be-chainloaded bootloader.
+
+## Protocol
+
+The bootloader binary should be an ELF file of the appropriate type
+(32- or 64-bit). It should contain the standard Xen ELF notes allowing
+it to be loaded by the Xen toolstack domain builder (TBD: Reference).
+
+## Path
+
+The second stage bootloader should be installed into the guest
+filesystem as:
+
+ * `/boot/xen/pvboot-<ARCH>.elf`
+
+Where `<ARCH>` is the first element of the GNU triplet e.g. one of:
+
+ * i386 (nb only i386, not i686 etc), corresponding to the Xen
+   x86\_32(p) arch;
+ * x86\_64, corresponding to the Xen x86\_64 arch;
+
+It is allowable for `/boot` to be a separate filesystem from `/` and
+therefore stage 1 bootloaders should search
+`/boot/xen/pvboot-<ARCH>.elf` and `/xen/pvboot-<ARCH>.elf` (in that
+order). The `xen` directory should be on the same filesystem as /boot
+and therefore it is not necessary to search for /pvboot-<ARCH>.elf.
+
+It is not in general possible under Xen for a bootloader to boot a
+kernel of a different width from itself, and this extends to
+chainloading from a stage one. Therefore it is permissible to have
+both `/boot/xen/pvboot-i386.elf` and `/boot/xen/pvboot-x86\_64.elf`
+present in a guest to be used by the appropriate stage 1 (e.g. for
+systems with 32-bit userspace and an optional 64-bit kernel).