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 16:35, 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.
> 
> This fixes Xen incorrectly reporting:
> 
> (XEN) Video information:
> (XEN)  VGA is text mode 80x25, font 8x16
> 
> When booted as a PVH guest.

And what effect does this have on a bare-metal boot with no-real-mode in use?
The default on x86 hardware still is that in the absence of other information,
a VGA of some kind can be assumed to be there. Yes, there are headless
systems, but better assume VGA is there when there's not than the other way
around.

What I would have expected is for the PVH boot path to clear boot_vid_info.

Jan