When barrier_nospec() was added the defintion was copied from the
one used to synchronise rdtsc.
On very old cpu rdtsc was a synchronising instruction.
When this change X86_FEATURE_LFENCE_RDTSC (and a MFENCE copy) were
(probably) added so lflence/mfence could be added to synchronise rdtsc.
For old cpu (I think the code checks XMM2) no barrier was added.
I'm not sure why that code was used for barrier_nospec().
I'm sure it should actually be rmb() with the fallback to a
locked memory access on old cpu.
In any case all x86-64 cpu support XMM2 and lfence so there is
to point using alternative().
Separate the 32bit and 64bit definitions but leave the barrier
missing on old 32bit cpu.
Signed-off-by: David Laight <david.laight.linux@xxxxxxxxx>
---
arch/x86/include/asm/barrier.h | 6 +++++-
1 file changed, 5 insertions(+), 1 deletion(-)
diff --git a/arch/x86/include/asm/barrier.h b/arch/x86/include/asm/barrier.h
index 7b44b3c4cce1..7eecce9bf4fe 100644
--- a/arch/x86/include/asm/barrier.h
+++ b/arch/x86/include/asm/barrier.h
@@ -45,7 +45,11 @@
__mask; })
/* Prevent speculative execution past this barrier. */
-#define barrier_nospec() alternative("", "lfence", X86_FEATURE_LFENCE_RDTSC)
+#ifdef CONFIG_X86_32
+#define barrier_nospec() alternative("", "lfence", X86_FEATURE_XMM2)
+#else
+#define barrier_nospec() __rmb()
+#endif
#define __dma_rmb() barrier()
#define __dma_wmb() barrier()
--
2.39.5
