Bug#776450: Xen PVH support for grub-xen in Buster

[Hans van Kranenburg]

On 1/6/19 11:28 AM, Colin Watson wrote:
> On Sun, Jan 06, 2019 at 01:45:53AM +0100, Hans van Kranenburg wrote:
>> Hm, so I have a PV (or PVH) domU in a test environment here, and I tried
>> to use it in the Debian way of booting it.
>>
>> If I install grub-xen-bin inside the domU then I get some stuff, but not
>> update-grub. When I just also install grub2 it drags in more stuff, and
>> then I end up with update-grub and a /boot/grub/grub.cfg file as result.
> 
> For the record, you wanted grub-xen rather than grub-xen-bin.

Yes, indeed, you're right.

>> I just copied *.mod that I got from my grub build (with the make blah
>> shown before) into /usr/lib/grub/i386-xen_pvh of the domU,
> 
> I'd expect them to need to be in /boot/grub/i386-xen_pvh/ in order for
> the boot loader to read them; grub-install copies modules there.
> (However, a standalone image normally shouldn't need to read modules
> from the guest anyway.)
> 
>> When trying to start that, I get:
>>
>> Reading (memdisk)/boot/grub/grub.cfg
>>
>> and then it exits back to the dom0 prompt and the domU is gone.
> 
> Seeing (memdisk) there suggests that $root is wrong.  I think that
> putting your bootstrap configuration file in /boot/grub/grub.cfg in the
> memdisk is a recipe for confusion; grub-xen-host puts it in /grub.cfg
> instead, and generally hooks things up a bit differently.  You can
> inspect debian/rules to see what it does.

Ok, I've read your explanation in the other email, I reread the xen wiki
page, and now is probably the moment to go back to where I started,
reading debian/rules, and see if I understand it a bit better now.

So, if I go to my upstream build directory and follow the same recipe:

-$ mkdir -p grub_dir/boot/grub
-$ sed -e "s/@@PVBOOT_ARCH@@/i386-xen_pvh/" <
~/path/to/debian/grub/packaging/debian/grub-xen-host_grub.cfg >
grub_dir/grub.cfg
-$ tar -cf - -C grub_dir grub.cfg > grub_memdisk
-$ ./grub-mkimage -O i386-xen_pvh -c
~/build/grub/grub/debian/grub-xen-host_grub-bootstrap.cfg -d
./grub-core/ ./grub-core/*.mod -m grub_memdisk -o grub-i386-xen_pvh.bin

Now I scp this grub-i386-xen_pvh.bin to my Xen dom0 and use it in the
guest config:

---- >8 ----
kernel = "/root/grub-i386-xen_pvh.bin"
type = "pvh"
---- >8 ----

Now I start with xl create -c and voila, a blue grub menu, a countdown
of 5 seconds and the whole thing starts correctly!

To confirm:

-# dmesg |grep PVH
[    0.192293] Booting paravirtualized kernel on Xen PVH

I've been looking at the modules in the domU and even had copied
everything into /boot/grub/i386-xen_pvh manually, but it seems this is
not used at all. It also works if I remove all of that again. So I'm
still wondering what that's for. Maybe the amount of information and
different scenarios and routes that can be chosen is a little bit too
much for me to all grasp at once.

> Anyway, this sounds close enough that we can probably go for it and see
> where it lands :-)

Well, proof of concept (tm) is a great success already.

Hans