From: Andi Kleen <ak@linux.intel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: David Laight <david.laight.linux@gmail.com>,
x86@kernel.org, linux-kernel@vger.kernel.org,
Thomas Gleixner <tglx@linutronix.de>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
Dave Hansen <dave.hansen@linux.intel.com>,
"H. Peter Anvin" <hpa@zytor.com>,
Catalin Marinas <catalin.marinas@arm.com>,
Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
Josh Poimboeuf <jpoimboe@redhat.com>,
Dan Williams <dan.j.williams@intel.com>,
linux-arch@vger.kernel.org, Kees Cook <keescook@chromium.org>,
kernel-hardening@lists.openwall.com
Subject: Re: [PATCH 1/1] x86: In x86-64 barrier_nospec can always be lfence
Date: Sun, 9 Feb 2025 20:29:42 -0800 [thread overview]
Message-ID: <Z6mAtkG9DnDDNFvn@tassilo> (raw)
In-Reply-To: <CAHk-=wiSnNEWsvDariBQ4O-mz7Nc7LbkdKUQntREVCFWiMe9zw@mail.gmail.com>
> So on x86, both read and write barriers are complete no-ops, because
> all reads are ordered, and all writes are ordered. So those only need
> compiler barriers to guarantee that the compiler itself doesn't
> re-order them.
>
> (Side note: earlier reads are also guaranteed to happen before later
> writes, so it's really only writes that can be delayed past reads, but
> we don't haev a barrier for that situation anyway. Also note that all
> of this is not "real" ordering, but only a guarantee that the
> user-visible semantics are AS IF they were actually ordered - if
> things are local in cache, ordering doesn't matter because no external
> CPU can *see* what the ordering was).
However in the local case *FENCE still orders, so it's actually not a
nop. Just normally you can't tell the difference in ordering semantics,
but it's visible in side effects like RDTSC.
-Andi
prev parent reply other threads:[~2025-02-10 4:30 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2025-02-09 19:10 [PATCH 1/1] x86: In x86-64 barrier_nospec can always be lfence David Laight
2025-02-09 19:32 ` Linus Torvalds
2025-02-09 21:40 ` David Laight
2025-02-09 21:57 ` Linus Torvalds
2025-02-10 1:09 ` Rik van Riel
2025-02-10 2:15 ` H. Peter Anvin
2025-02-10 4:29 ` Andi Kleen [this message]
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