BGRT preservation is now enabled by default to fix ACPI corruption
for desktop/workstation systems (similar to ESRT).

As described in the task:
https://github.com/QubesOS/qubes-issues/issues/10764

Add an opt-out parameter to allow disabling BGRT preservation on
systems where the ~1MB memory overhead is not desired.

The opt-out is implemented through two boot paths with early parsing
during the EFI boot phase before preservation runs:

1. xen.efi direct boot: '-nobgrt' command line option (parsed in
   efi_start())
2. Multiboot2 (GRUB): 'efi=no-bgrt' peeked from mb2 cmdline tag
   using get_option() in efi_multiboot2()

The flag is checked at the start of efi_preserve_bgrt_img() to
skip preservation entirely when disabled. Status logging indicates
whether preservation was disabled, succeeded, or failed.

Usage:
  Default: BGRT preserved automatically
  xen.efi: Add '-nobgrt' option
  GRUB/MB2: Add 'efi=no-bgrt' to Xen command line

Signed-off-by: Soumyajyotii Ssarkar <soumyajyotisarkar23@gmail.com>
---
 xen/arch/x86/efi/efi-boot.h |  3 +++
 xen/common/efi/boot.c       | 11 ++++++++++-
 2 files changed, 13 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/efi/efi-boot.h b/xen/arch/x86/efi/efi-boot.h
index 0547d845cd..6c986cf6c0 100644
--- a/xen/arch/x86/efi/efi-boot.h
+++ b/xen/arch/x86/efi/efi-boot.h
@@ -897,6 +897,9 @@ void __init efi_multiboot2(EFI_HANDLE ImageHandle,
         efi_arch_edid(gop_handle);
     }

+    if ( cmdline && get_option(cmdline, "efi=no-bgrt") )
+        opt_bgrt_disabled = true;
+
     efi_arch_edd();
     efi_arch_cpu();

diff --git a/xen/common/efi/boot.c b/xen/common/efi/boot.c
index 68e06d707c..dc46e783f3 100644
--- a/xen/common/efi/boot.c
+++ b/xen/common/efi/boot.c
@@ -170,6 +170,7 @@ static SIMPLE_TEXT_OUTPUT_INTERFACE *__initdata StdErr;

 static UINT32 __initdata mdesc_ver;
 static bool __initdata map_bs;
+static bool __initdata opt_bgrt_disabled = false;

 static struct file __initdata cfg;
 static struct file __initdata kernel;
@@ -825,6 +826,9 @@ static void __init efi_preserve_bgrt_img(void)

     bgrt_info.preserved = false;

+    if ( opt_bgrt_disabled )
+        return;
+
     bgrt = efi_get_bgrt();
     if ( !bgrt )
     {
@@ -1582,6 +1586,8 @@ void EFIAPI __init noreturn efi_start(EFI_HANDLE ImageHandle,
                     base_video = true;
                 else if ( wstrcmp(ptr + 1, L"mapbs") == 0 )
                     map_bs = true;
+                else if ( wstrcmp(ptr + 1, L"nobgrt") == 0 )
+                    opt_bgrt_disabled = true;
                 else if ( wstrncmp(ptr + 1, L"cfg=", 4) == 0 )
                     cfg_file_name = ptr + 5;
                 else if ( i + 1 < argc && wstrcmp(ptr + 1, L"cfg") == 0 )
@@ -1592,6 +1598,7 @@ void EFIAPI __init noreturn efi_start(EFI_HANDLE ImageHandle,
                     PrintStr(L"Xen EFI Loader options:\r\n");
                     PrintStr(L"-basevideo   retain current video mode\r\n");
                     PrintStr(L"-mapbs       map EfiBootServices{Code,Data}\r\n");
+                    PrintStr(L"-nobgrt      disable BGRT preservation\r\n");
                     PrintStr(L"-cfg=<file>  specify configuration file\r\n");
                     PrintStr(L"-help, -?    display this help\r\n");
                     blexit(NULL);
@@ -1916,7 +1923,9 @@ void __init efi_bgrt_status_info(void)
     if ( !efi_enabled(EFI_BOOT) )
         return;

-    if ( bgrt_info.preserved )
+    if ( opt_bgrt_disabled )
+        printk(XENLOG_INFO "EFI: BGRT preservation disabled\n");
+    else if ( bgrt_info.preserved )
     {
         printk(XENLOG_INFO "EFI: BGRT image preserved: %lu KB\n",
                bgrt_info.size / 1024);
--
2.53.0