[x86 setup] Correct the SMAP check for INT 0x15, AX=0xe820

The e820 probe code was checking %edx, not %eax, for the SMAP
signature on return.  This worked on *almost* all systems, since %edx
still contained SMAP from the call on entry, but on a handful of
systems it failed -- plus, we would have missed real mismatches.

The error output is "=d" to make sure gcc knows %edx is clobbered
here.

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

1 files changed, 6 insertions, 3 deletions

diff --git a/arch/i386/boot/memory.c b/arch/i386/boot/memory.c
index bccaa1cf6645..378353956b5d 100644
--- a/arch/i386/boot/memory.c
+++ b/arch/i386/boot/memory.c
@@ -28,11 +28,14 @@ static int detect_memory_e820(void)
 
         do {
                 size = sizeof(struct e820entry);
-                id = SMAP;
+
+                /* Important: %edx is clobbered by some BIOSes,
+                   so it must be either used for the error output
+                   or explicitly marked clobbered. */
                 asm("int $0x15; setc %0"
-                    : "=am" (err), "+b" (next), "+d" (id), "+c" (size),
+                    : "=d" (err), "+b" (next), "=a" (id), "+c" (size),
                       "=m" (*desc)
-                    : "D" (desc), "a" (0xe820));
+                    : "D" (desc), "d" (SMAP), "a" (0xe820));
 
                 /* Some BIOSes stop returning SMAP in the middle of
                    the search loop.  We don't know exactly how the BIOS