[XS] Change in coreboot[main]: commonlib/include/commonlib: Add barrier in write_at_ble8()

[Patrick Rudolph (Code Review)]

Patrick Rudolph has uploaded this change for review. ( https://review.coreboot.org/c/coreboot/+/85855?usp=email )

Change subject: commonlib/include/commonlib: Add barrier in write_at_ble8()
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commonlib/include/commonlib: Add barrier in write_at_ble8()

With the introduction of the stack canary breakpoint QEMU uncovered
a different bug within coreboot. Currently the compiler optimizes
over aggressively inline functions and memory stores.

That also affects write_at_ble8(), which is supposed to store a
single byte at time. The compiler however optimizes multiple byte
stores into a single wider (and possibly unaligned) store operation.

Make sure that the compiler does not optimize multiple calls to
write_at_ble8() by adding a memory barrier.

Fixes a strange bug in QEMU where it triggers the DEBUG INT
handler on unaligned 16-bit stores in the first 4KiB of memory.
Aligned stores and store outside of the first 4KiB do not dispatch
the DEBUG INT handler.

Change-Id: Ibbc661235a38c7f7540b656a67f067c3e51105d1
Signed-off-by: Patrick Rudolph 
---
M src/commonlib/include/commonlib/endian.h
1 file changed, 5 insertions(+), 0 deletions(-)

git pull ssh://review.coreboot.org:29418/coreboot refs/changes/55/85855/1

diff --git a/src/commonlib/include/commonlib/endian.h b/src/commonlib/include/commonlib/endian.h
index 84c90b4..d0280b1 100644
--- a/src/commonlib/include/commonlib/endian.h
+++ b/src/commonlib/include/commonlib/endian.h
@@ -25,6 +25,11 @@
 static inline void write_ble8(void *dest, uint8_t val)
 {
    *(uint8_t *)dest = val;
+	/*
+	 * Prevent compilers from optimizing multiple write_ble8() into
+	 * a single unaligned store operation by adding a memory barrier.
+	 */
+	__asm__ __volatile__("" : : : "memory");
 }

static inline void write_at_ble8(void *dest, uint8_t val, size_t offset)