vt/vga: ignore ACPI_FADT_NO_VGA unless running virtualized

There's too many broken hardware out there that wrongly has the
ACPI_FADT_NO_VGA bit set. Ignore it unless running as a virtualized
guest, as then the expectation would be that the hypervisor does
provide correct ACPI tables.

Reviewed by: emaste, 0mp, eugen
Sponsored by: Citrix Systems R&D
PR: 230172

(cherry picked from commit 0518832011caba1e9dcee054d7884797ed8a74c2)

2 files changed, 9 insertions, 2 deletions

diff --git a/share/man/man4/vt.4 b/share/man/man4/vt.4
index e057ca6e0351..9c8019925544 100644
--- a/share/man/man4/vt.4
+++ b/share/man/man4/vt.4
@@ -24,7 +24,7 @@
 .\"
 .\" $FreeBSD$
 .\"
-.Dd March 4, 2022
+.Dd March 17, 2022
 .Dt "VT" 4
 .Os
 .Sh NAME
@@ -204,6 +204,9 @@ graphics mode.
 Set to 1 to force the usage of the VGA driver regardless of whether
 ACPI IAPC_BOOT_ARCH signals no VGA support.
 Can be used to workaround firmware bugs in the ACPI tables.
+Note no VGA support is only acknowledged when running virtualized.
+There is too many broken firmware that wrongly reports no VGA support on
+physical hardware.
 .It Va kern.vty
 Set this value to
 .Ql vt
diff --git a/sys/dev/vt/hw/vga/vt_vga.c b/sys/dev/vt/hw/vga/vt_vga.c
index bac8f3eb7b97..d457f8c7b31d 100644
--- a/sys/dev/vt/hw/vga/vt_vga.c
+++ b/sys/dev/vt/hw/vga/vt_vga.c
@@ -1261,7 +1261,11 @@ vga_acpi_disabled(void)
 	uint16_t flags;
 	int ignore;
 
-	ignore = 0;
+	/*
+	 * Ignore the flag on real hardware: there's a lot of buggy firmware
+	 * that will wrongly set it.
+	 */
+	ignore = (vm_guest == VM_GUEST_NO);
 	TUNABLE_INT_FETCH("hw.vga.acpi_ignore_no_vga", &ignore);
 	if (ignore || !acpi_get_fadt_bootflags(&flags))
  		return (false);