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

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

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.

Reported-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Roger Pau Monné <roger.pau@xxxxxxxxxx>
---
 xen/arch/x86/boot/video.S | 7 +------
 1 file changed, 1 insertion(+), 6 deletions(-)

diff --git a/xen/arch/x86/boot/video.S b/xen/arch/x86/boot/video.S
index a4b25a3b34d1..a51de04a024e 100644
--- a/xen/arch/x86/boot/video.S
+++ b/xen/arch/x86/boot/video.S
@@ -994,12 +994,7 @@ name_bann:      .asciz  "Video adapter: "
 force_size:     .word   0       # Use this size instead of the one in BIOS vars
 
 GLOBAL(boot_vid_info)
-        .byte   0, 0    /* orig_x, orig_y */
-        .byte   3       /* text mode 3    */
-        .byte   80, 25  /* 80x25          */
-        .byte   1       /* isVGA          */
-        .word   16      /* 8x16 font      */
-        .space  BVI_size - BVI_capabilities
+        .space  BVI_size
 GLOBAL(boot_edid_info)
         .fill   128,1,0x13
 GLOBAL(boot_edid_caps)
-- 
2.44.0