Re: black screen troubles trying to boot with Xen Hypervisor grub option

[Pierre-Philipp Braun <pbraun@xxxxxxxxxxxx>]

> While digging in to what's going on I also noticed that the base 
> hypervisor entry has a conditional branch checking $grub_platform so I 
> added an echo to figure out which branch it's picking, and it turns out 
> that in my case $grub_platform isn't "pc" and isn't empty but rather is 
> "efi" so it does the "no-real-mode edd=off" options.  When I read about 
> them in the documentation says that no-real-mode should only be used for 
> debugging because it prevents vga from working, and I got excited 
> because I thought "and therefore black screen!" but when I force it to 
> use no options, like the "pc" branch does, it still gets black screen.
About no-real-mode edd=off, you might be right.  But to test without it, I 
would go with my own grub.cfg by overwriting it altogether, to keep it simple 
(keep insmod part_gpt as for EFI).  I find interesting that there is usually 
`insmod all_video` or a call to function `load_video` with the valid entries 
(EFI capable), but those are missing in the XEN entries.

So this is the relevant part:

menuentry 'Debian GNU/Linux, with Xen hypervisor' --class debian --class 
gnu-linux --class gnu --class os --class xen $menuentry_id_option 
'xen-gnulinux-simple-0e276b18-54a4-4ff9-a770-1bcdd946ea70' {
        insmod part_gpt
        insmod ext2
        set root='hd0,gpt1'
        if [ x$feature_platform_search_hint = xy ]; then
          search --no-floppy --fs-uuid --set=root --hint-bios=hd0,gpt1 
--hint-efi=hd0,gpt1 --hint-baremetal=ahci0,gpt1  
1dd68754-c4eb-4011-98d4-6df73f99eb81
        else
          search --no-floppy --fs-uuid --set=root 
1dd68754-c4eb-4011-98d4-6df73f99eb81
        fi
        echo    'Loading Xen 4.17-amd64 ...'
        if [ "$grub_platform" = "pc" -o "$grub_platform" = "" ]; then
            xen_rm_opts=
        else
            xen_rm_opts="no-real-mode edd=off"
        fi
        multiboot2      /xen-4.17-amd64.gz placeholder   ${xen_rm_opts}
        echo    'Loading Linux 6.1.0-10-amd64 ...'
        module2 /vmlinuz-6.1.0-10-amd64 placeholder 
root=UUID=0e276b18-54a4-4ff9-a770-1bcdd946ea70 ro  quiet
        echo    'Loading initial ramdisk ...'
        module2 --nounzip   /initrd.img-6.1.0-10-amd64
}

You could try add load_video in top of that and reboot (and without 
re-generating the entire thing with update-grub).

Besides, note there used to be an issue on Debian systems while setting up XEN. 
 One had to elevate the boot priority.

        dpkg-divert --divert /etc/grub.d/08_linux_xen --rename 
/etc/grub.d/20_linux_xen
        update-grub

However I am not sure this is still the case.  Also, to get the login prompt, 
some console needs to point hvc0, and I don't see any specific console= 
argument to the kernel here.  You can also disable the casual serial console 
prompt altogether, since you don't have any.

        systemctl disable serial-getty@ttyS0

It's usually harder to setup XEN on EFI systems compared to legacy MBR boot 
process.  The easy way would be to enable CSM in your firmware and go MBR.