[PATCH 3/3] x86/efi: Add opt-out mechanism for BGRT preservation

[Soumyajyotii Ssarkar <soumyajyotisarkar23@xxxxxxxxx>]

As described in the task, BGRT preservation is now enabled by default to fix 
ACPI corruption
for desktop/workstation systems (similar to ESRT).

Add an opt-out parameter 'efi=no-bgrt' to allow disabling BGRT
preservation on systems where the ~1MB memory overhead is not
desired.

The parameter is parsed during normal Xen boot (not during EFI
phase), so it only affects diagnostic logging. The opt-out flag
is checked at the start of efi_preserve_bgrt_img().

Usage:
  Default: BGRT preserved automatically
  Opt-out: Add 'efi=no-bgrt' in Xen command line

Signed-off-by: Soumyajyotii Ssarkar <soumyajyotisarkar23@xxxxxxxxx>
---
 xen/common/efi/boot.c | 14 +++++++++++++-
 1 file changed, 13 insertions(+), 1 deletion(-)

diff --git a/xen/common/efi/boot.c b/xen/common/efi/boot.c
index 1e3489e902..b735eac6b2 100644
--- a/xen/common/efi/boot.c
+++ b/xen/common/efi/boot.c
@@ -167,6 +167,7 @@ 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;
@@ -868,6 +869,9 @@ static void __init efi_preserve_bgrt_img(EFI_SYSTEM_TABLE 
*SystemTable)
     bgrt_debug_info.preserved = false;
     bgrt_debug_info.failure_reason = NULL;

+    if ( opt_bgrt_disabled )
+        return;
+
     bgrt = find_bgrt_table(SystemTable);
     if ( !bgrt )
     {
@@ -1873,6 +1877,10 @@ static int __init cf_check parse_efi_param(const char *s)
             else
                 __clear_bit(EFI_RS, &efi_flags);
         }
+        else if ( (ss - s) == 7 && !memcmp(s, "no-bgrt", 7) )
+        {
+            opt_bgrt_disabled = true;
+        }
         else if ( (ss - s) > 5 && !memcmp(s, "attr=", 5) )
         {
             if ( !cmdline_strcmp(s + 5, "uc") )
@@ -1968,7 +1976,11 @@ void __init efi_init_memory(void)
     if ( !efi_enabled(EFI_BOOT) )
         return;

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