[Xen-devel] GRUB Xen PVH chainloading

[Colin Watson <cjwatson@xxxxxxxxxx>]

Hi,

I'm working on integrating the recently-merged PVH support for GRUB into
the Debian packages.  As a result I find myself thinking about how to
handle the two-stage boot loader scheme that our packages currently
implement for PV.  I think that it would not be very hard to do this for
PVH in the manner outlined below, but my x86 asm skills aren't quite up
to some of the work needed in GRUB.  Assuming that nobody sees any
obvious holes in this, does anyone fancy giving it a go?

Background
----------

Around the time PV support was implemented in GRUB 2, we put together a
scheme to minimise the coupling between the guest configuration file on
the host and the boot loader configuration in the guest.  The scheme and
its rationale are described here:

  
https://wiki.xen.org/wiki/PvGrub2#Chainloading_guest_pvgrub2_from_domain_0_pvgrub2

Essentially the same rationale applies to the PVH case: it should be
possible for the guest to declare its own booting arrangements (though
of course some hosts may wish to just provide a grub.cfg and handle all
that on the host side), and this should be decoupled from the GRUB image
provided by the host as far as possible in order to minimise
compatibility issues.

There seems to be no obstacle to this in principle: just as a PV boot
loader can chainload another one from the guest's disk, so too could a
PVH boot loader chainload another one from the guest's disk.

What needs to be done
---------------------

GRUB needs to support chainloading another PVH boot loader.  I think
this involves observing the existence of the XEN_ELFNOTE_PHYS32_ENTRY
note and following the machine state rules for the domain builder here:

  https://xenbits.xen.org/docs/unstable-staging/misc/pvh.html

(I had a brief go at implementing this, but foundered on my fairly
minimal understanding of GRUB's relocator/boot code.)

We need to define a modification to
https://xenbits.xen.org/docs/unstable-staging/misc/x86-xenpv-bootloader.html
for PVH boot loaders.  I suggest the obvious: a second-stage PVH boot
loader should be installed into the guest filesystem as
/boot/xen/pvhboot-<ARCH>.elf, and otherwise things generally behave the
same way.  I'd be happy to draft a patch to the protocol specification
once a proof-of-concept exists.

The as-yet-unmerged GRUB patch to support the existing PV boot protocol
(https://salsa.debian.org/grub-team/grub/blob/master/debian/patches/grub-install-pvxen-paths.patch)
needs to be extended to support the amended protocol.  This is trivial
given the above.

Thanks,

-- 
Colin Watson                                       [cjwatson@xxxxxxxxxx]

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