linux-kernel - Re: [RFC PATCH 00/16] x86/split_lock: Enable #AC exception for split locked accesses

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Message-ID: <20180621193738.GA13636@worktop.programming.kicks-ass.net>
Date:   Thu, 21 Jun 2018 21:37:38 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Fenghua Yu <fenghua.yu@...el.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...e.hu>,
        "H. Peter Anvin" <hpa@...ux.intel.com>,
        Ashok Raj <ashok.raj@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Rafael Wysocki <rafael.j.wysocki@...el.com>,
        Tony Luck <tony.luck@...el.com>,
        Alan Cox <alan@...ux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@...el.com>,
        Arjan van de Ven <arjan@...radead.org>,
        linux-kernel <linux-kernel@...r.kernel.org>, x86 <x86@...nel.org>
Subject: Re: [RFC PATCH 00/16] x86/split_lock: Enable #AC exception for split
 locked accesses

On Sun, May 27, 2018 at 08:45:49AM -0700, Fenghua Yu wrote:
> Currently we can trace split lock event counter for debug purpose. But

How? A while ago I actually tried that, but I could not find a suitable
perf event.

> Intel introduces mechanism to detect split lock via alignment
> check exception in Tremont and other future processors. If split lock is
> from user process, #AC handler can kill the process or re-execute faulting
> instruction depending on configuration. 

Ideally it would #AC any unaligned (implied) LOCK prefix instruction,
not just across lines.

> To detect split lock, a new control bit (bit 29) in per-core TEST_CTL
> MSR 0x33 will be introduced in future x86 processors. When the bit 29
> is set, the processor causes #AC exception for split locked accesses at
> all CPL.

Per-Core is really unfortunate, but better than nothing.