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

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Message-ID: <15734b32cecddde7905d3a97005a0c883383cc74.camel@surriel.com>
Date: Sun, 09 Feb 2025 20:09:51 -0500
From: Rik van Riel <riel@...riel.com>
To: Linus Torvalds <torvalds@...ux-foundation.org>, David Laight
	 <david.laight.linux@...il.com>
Cc: 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>, 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, "Paul
 E.McKenney" <paulmck@...nel.org>
Subject: Re: [PATCH 1/1] x86: In x86-64 barrier_nospec can always be lfence

On Sun, 2025-02-09 at 13:57 -0800, Linus Torvalds wrote:
> 
> So on x86, both read and write barriers are complete no-ops, because
> all reads are ordered, and all writes are ordered.

Given that this thread started with a reference
to rdtsc, it may be worth keeping in mind that
rdtsc reads themselves do not always appear to
be ordered.

Paul and I spotted some occasionaly "backwards
TSC values" from the CSD lock instrumentation code, 
which went away when using ordered TSC reads:

https://lkml.iu.edu/hypermail/linux/kernel/2410.1/03202.html

I guess maybe a TSC read does not follow all the same
rules as a memory read, sometimes?

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