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

[Patrick Rudolph (Code Review)]

Patrick Rudolph has uploaded a new patch set (#3). ( 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.

This can be seen in the emited assembly code of write_le16(), as used
to install the EBDA:
 401348a:       66 c7 04 25 13 04 00    movw   $0x400,0x413
 4013491:       00 00 04

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

The emitted assembly code of the same function changes to:
 401394c:       c6 04 25 13 04 00 00    movb   $0x0,0x413
 4013953:       00
 4013954:       c6 04 25 14 04 00 00    movb   $0x4,0x414
 401395b:       04

Fixes a strange bug in QEMU where it triggers the DEBUG breakpoint
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 breakpoint 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/55/85855/3