[bios] Make uses of REAL_CODE() and PHYS_CODE() 64-bit clean

On a 64-bit CPU, any modification of a register by 32-bit or 16-bit
code will destroy the invisible upper 32 bits of the corresponding
64-bit register.  For example: a 32-bit "pushl %eax" followed by a
"popl %eax" will zero the upper half of %rax.  This differs from the
treatment of upper halves of 32-bit registers by 16-bit code: a
"pushw %ax" followed by a "popw %ax" will leave the upper 16 bits of
%eax unmodified.

Inline assembly generated using REAL_CODE() or PHYS_CODE() will
therefore have to preserve the upper halves of all registers, to avoid
clobbering registers that gcc expects to be preserved.

Output operands from REAL_CODE() and PHYS_CODE() assembly may
therefore contain undefined values in the upper 32 bits.

Fix by using explicit variable widths (e.g. uint32_t) for
non-discarded output operands, to ensure that undefined values in the
upper 32 bits of 64-bit registers are ignored.

Signed-off-by: Michael Brown <mcb30@ipxe.org>

1 files changed, 2 insertions, 2 deletions

diff --git a/src/arch/x86/image/nbi.c b/src/arch/x86/image/nbi.c
index 99046144d..b691bee20 100644
--- a/src/arch/x86/image/nbi.c
+++ b/src/arch/x86/image/nbi.c
@@ -241,7 +241,7 @@ static int nbi_process_segments ( struct image *image,
  */
 static int nbi_boot16 ( struct image *image, struct imgheader *imgheader ) {
 	int discard_D, discard_S, discard_b;
-	int rc;
+	int32_t rc;
 
 	DBGC ( image, "NBI %p executing 16-bit image at %04x:%04x\n", image,
 	       imgheader->execaddr.segoff.segment,
@@ -283,7 +283,7 @@ static int nbi_boot32 ( struct image *image, struct imgheader *imgheader ) {
 		0
 	};
 	int discard_D, discard_S, discard_b;
-	int rc;
+	int32_t rc;
 
 	DBGC ( image, "NBI %p executing 32-bit image at %lx\n",
 	       image, imgheader->execaddr.linear );