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

[Roger Pau Monné <roger.pau@xxxxxxxxxx>]

On Tue, Apr 02, 2024 at 11:49:20AM +0200, Jan Beulich wrote:
> 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.

But that contradicts the text of the 'no-real-mode' option, which
explicitly notes:

"Do not execute real-mode bootstrap code when booting Xen. This option
should not be used except for debugging. It will effectively disable
the vga option, which relies on real mode to set the video mode."

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

Well, my intention was to fix both PVH and also make the
implementation of the 'no-real-mode' option consistent with the
documentation.

Thanks, Roger.