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Message-Id: <20250209220222.212835-1-david.laight.linux@gmail.com>
Date: Sun, 9 Feb 2025 22:02:22 +0000
From: David Laight <david.laight.linux@...il.com>
To: x86@...nel.org,
linux-kernel@...r.kernel.org,
Linus Torvalds <torvalds@...ux-foundation.org>
Cc: David Laight <david.laight.linux@...il.com>,
Thomas Gleixner <tglx@...utronix.de>,
Ingo Molnar <mingo@...hat.com>,
Borislav Petkov <bp@...en8.de>,
Dave Hansen <dave.hansen@...ux.intel.com>,
"H. Peter Anvin" <hpa@...or.com>,
Catalin Marinas <catalin.marinas@....com>,
Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
Josh Poimboeuf <jpoimboe@...hat.com>,
Andi Kleen <ak@...ux.intel.com>,
Dan Williams <dan.j.williams@...el.com>,
linux-arch@...r.kernel.org,
Kees Cook <keescook@...omium.org>,
kernel-hardening@...ts.openwall.com
Subject: [PATCH v2 1/1] x86: In x86-64 barrier_nospec can always be lfence
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 lfence/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().
Or why rdtsc ended up being synchronised by barrier_nospec().
lfence is the right instruction (well as good as you get).
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@...il.com>
v2: use a explicit lfence rather than __rmb().
Update commit message text w.r.t rdtsc.
Signed-off-by: David Laight <david.laight.linux@...il.com>
---
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..b9af75624cf5 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() asm volatile("lfence":::"memory")
+#endif
#define __dma_rmb() barrier()
#define __dma_wmb() barrier()
--
2.39.5
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