[PATCH v3 1/3] x86/fpu: Make XCR0 accessors immune to unwanted compiler reordering

[Chang S. Bae]

Some old GCC versions (4.9.x and 5.x) have an issue that incorrectly
reordering volatile asm statements with each other [1]. While this bug was
fixed on later versions (8.1, 7.3, and 6.5), and the kernel's current XCR0
read/write do not appear to be impacted, it is preventive to ensure them on
the program order.

Have a memory clobber for write to prevent caching/reordering memory
accesses across other XCR0 writes. A dummy operand with an arbitrary
address can prevent the compiler from reordering with other writes. Add the
dummy operand for read as used for other accessors in aa5cacdc29d
("x86/asm: Replace __force_order with a memory clobber").

[1] https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82602
Signed-off-by: Chang S. Bae <chang.seok.bae@xxxxxxxxx>
Cc: x86@xxxxxxxxxx
Cc: linux-kernel@xxxxxxxxxxxxxxx
---
Changes from v2:
* Add as a new patch (Dave Hansen).
---
 arch/x86/include/asm/fpu/xcr.h | 8 ++++++--
 1 file changed, 6 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/fpu/xcr.h b/arch/x86/include/asm/fpu/xcr.h
index 9656a5bc6fea..9b513e7c0161 100644
--- a/arch/x86/include/asm/fpu/xcr.h
+++ b/arch/x86/include/asm/fpu/xcr.h
@@ -2,6 +2,8 @@
 #ifndef _ASM_X86_FPU_XCR_H
 #define _ASM_X86_FPU_XCR_H
 
+#include <asm/special_insns.h>
+
 #define XCR_XFEATURE_ENABLED_MASK	0x00000000
 #define XCR_XFEATURE_IN_USE_MASK	0x00000001
 
@@ -9,7 +11,8 @@ static inline u64 xgetbv(u32 index)
 {
 	u32 eax, edx;
 
-	asm volatile("xgetbv" : "=a" (eax), "=d" (edx) : "c" (index));
+	asm volatile("xgetbv" : "=a" (eax), "=d" (edx) : "c" (index),
+				__FORCE_ORDER);
 	return eax + ((u64)edx << 32);
 }
 
@@ -18,7 +21,8 @@ static inline void xsetbv(u32 index, u64 value)
 	u32 eax = value;
 	u32 edx = value >> 32;
 
-	asm volatile("xsetbv" :: "a" (eax), "d" (edx), "c" (index));
+	asm volatile("xsetbv" :: "a" (eax), "d" (edx), "c" (index)
+			      :	 "memory");
 }
 
 /*
-- 
2.17.1