From: Rik van Riel <riel@surriel.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
David Laight <david.laight.linux@gmail.com>
Cc: 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>,
Andi Kleen <ak@linux.intel.com>,
Dan Williams <dan.j.williams@intel.com>,
linux-arch@vger.kernel.org, Kees Cook <keescook@chromium.org>,
kernel-hardening@lists.openwall.com,
"Paul E.McKenney" <paulmck@kernel.org>
Subject: Re: [PATCH 1/1] x86: In x86-64 barrier_nospec can always be lfence
Date: Sun, 09 Feb 2025 20:09:51 -0500 [thread overview]
Message-ID: <15734b32cecddde7905d3a97005a0c883383cc74.camel@surriel.com> (raw)
In-Reply-To: <CAHk-=wiSnNEWsvDariBQ4O-mz7Nc7LbkdKUQntREVCFWiMe9zw@mail.gmail.com>
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?
--
All Rights Reversed.
next prev parent reply other threads:[~2025-02-10 1:10 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 [this message]
2025-02-10 2:15 ` H. Peter Anvin
2025-02-10 4:29 ` Andi Kleen
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