linux-kernel - Re: [PATCH 1/1] x86: In x86-64 barrier

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Message-ID: <Z6mAtkG9DnDDNFvn@tassilo>
Date: Sun, 9 Feb 2025 20:29:42 -0800
From: Andi Kleen <ak@...ux.intel.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: David Laight <david.laight.linux@...il.com>, x86@...nel.org,
	linux-kernel@...r.kernel.org, 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>,
	Dan Williams <dan.j.williams@...el.com>, linux-arch@...r.kernel.org,
	Kees Cook <keescook@...omium.org>,
	kernel-hardening@...ts.openwall.com
Subject: Re: [PATCH 1/1] x86: In x86-64 barrier_nospec can always be lfence

> 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