Re: [PATCH 2/2] x86/video: do not assume a video mode to be unconditionally present

[Jan Beulich <jbeulich@suse.com>]

On 28.03.2024 20:58, Andrew Cooper wrote:
> On 28/03/2024 3:35 pm, Roger Pau Monne wrote:
>> There's no reason to assume VGA text mode 3 to be unconditionally available.
>> With the addition of booting Xen itself in PVH mode there's a boot path that
>> explicitly short-circuits all the real-mode logic, including the VGA detection.
>>
>> Leave the default user selected mode as text mode 3 in boot_vid_mode, but do
>> not populate boot_vid_info with any default settings.  It will either be
>> populated by the real-mode video detection code, or left zeroed in case
>> real-mode code is skipped.
>>
>> Note that only PVH skips the real-mode portion of the boot trampoline,
>> otherwise the only way to skip it is to set `no-real-mode` on the command line,
>> and the description for the option already notes that VGA would be disabled as
>> a result of skipping real-mode bootstrap.
> 
> IIRC, Grub defaults to using no-real-mode for xen.efi.

That's our MB2 entry path which forces skip_realmode to 1. The xen.efi boot
path doesn't, but similarly skips entering real mode (retrieving desired
data via EFI protocols instead). Imo if to be retained, the above paragraph
would want extending some, to cover all the cases.

Jan