arch/x86,cpu/x86: Disable the %gs and %fs segments
The %fs and %gs segment are typically used to implement thread local
storage or cpu local storage. We don't currently use these in coreboot,
so there is no reason to map them. By setting the segment index to 0,
it disables the segment. If an instruction tries to read from one of
these segments an exception will be raised.
The end goal is to make cpu_info() use the %gs segment. This will remove
the stack alignment requirements and fix smm_do_relocation.
BUG=b:194391185, b:179699789
TEST=Boot guybrush to OS
Signed-off-by: Raul E Rangel <rrangel@chromium.org>
Change-Id: Iaa376e562acc6bd1dfffb7a23bdec82aa474c1d5
Reviewed-on: https://review.coreboot.org/c/coreboot/+/57860
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Eric Peers <epeers@google.com>
Reviewed-by: Arthur Heymans <arthur@aheymans.xyz>
diff --git a/src/cpu/x86/entry32.S b/src/cpu/x86/entry32.S
index ad4a7d0..215d601 100644
--- a/src/cpu/x86/entry32.S
+++ b/src/cpu/x86/entry32.S
@@ -37,8 +37,9 @@
movw %ax, %ds
movw %ax, %es
movw %ax, %ss
+ xor %ax, %ax /* zero out the gs and fs segment index */
movw %ax, %fs
- movw %ax, %gs
+ movw %ax, %gs /* Will be used for cpu_info */
/* Restore the BIST value to %eax */
movl %ebp, %eax