x86, setup: guard against pre-ACPI 3 e820 code not updating %ecx

Impact: BIOS bug safety

For pre-ACPI 3 BIOSes, pre-initialize the end of the e820 buffer just
in case the BIOS returns an unchanged %ecx but without actually
touching the ACPI 3 extended flags field.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>

1 files changed, 6 insertions, 0 deletions

diff --git a/arch/x86/boot/memory.c b/arch/x86/boot/memory.c
index d5d2360763dc..5054c2ddd1a0 100644
--- a/arch/x86/boot/memory.c
+++ b/arch/x86/boot/memory.c
@@ -31,6 +31,12 @@ static int detect_memory_e820(void)
         struct e820entry *desc = boot_params.e820_map;
         static struct e820_ext_entry buf; /* static so it is zeroed */
 
+        /*
+         * Set this here so that if the BIOS doesn't change this field
+         * but still doesn't change %ecx, we're still okay...
+         */
+        buf.ext_flags = 1;
+
         do {
                 size = sizeof buf;