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Re: [PATCH 2/2] x86/video: do not assume a video mode to be unconditionally present
- To: Roger Pau Monne <roger.pau@xxxxxxxxxx>
- From: Jan Beulich <jbeulich@xxxxxxxx>
- Date: Tue, 2 Apr 2024 11:49:20 +0200
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- Cc: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>, xen-devel@xxxxxxxxxxxxxxxxxxxx
- Delivery-date: Tue, 02 Apr 2024 09:49:24 +0000
- List-id: Xen developer discussion <xen-devel.lists.xenproject.org>
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
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