linux-kernel - RE: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by kernel parameter

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Date:   Thu, 12 Dec 2019 16:23:25 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Andy Lutomirski' <luto@...capital.net>,
        Peter Zijlstra <peterz@...radead.org>
CC:     "Luck, Tony" <tony.luck@...el.com>,
        Andy Lutomirski <luto@...nel.org>,
        "Yu, Fenghua" <fenghua.yu@...el.com>,
        Ingo Molnar <mingo@...nel.org>,
        "Thomas Gleixner" <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "Borislav Petkov" <bp@...en8.de>, H Peter Anvin <hpa@...or.com>,
        "Raj, Ashok" <ashok.raj@...el.com>,
        "Shankar, Ravi V" <ravi.v.shankar@...el.com>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        x86 <x86@...nel.org>, "Will Deacon" <will@...nel.org>
Subject: RE: [PATCH v10 6/6] x86/split_lock: Enable split lock detection by
 kernel parameter

From: Andy Lutomirski
> Sent: 12 December 2019 16:02
...
> MFENCE also implies LFENCE, and LFENCE is fairly slow despite having no architectural semantics other than blocking speculative
> execution. AFAICT, in the absence of side channels timing oddities, there is no code whatsoever that would be correct with LFENCE
> but incorrect without it. “Serialization” is, to some extent, a weaker example of this — MOV to CR2 is *much* slower than MFENCE or
> LOCK despite the fact that, as far as the memory model is concerned, it doesn’t do a whole lot more.

IIRC LFENCE does affect things when you are mixing non-temporal and/or write-combining
memory accesses.

I also thought there was a case where you needed to stop the speculative reads.
But can't remember why.

	David

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