Ian Campbell
250073f452
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
1.9 KiB
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-.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).