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Date:   Thu, 21 Jun 2018 22:36:56 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Fenghua Yu <fenghua.yu@intel.com>
Cc:     Thomas Gleixner <tglx@linutronix.de>, Ingo Molnar <mingo@elte.hu>,
        "H. Peter Anvin" <hpa@linux.intel.com>,
        Ashok Raj <ashok.raj@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Rafael Wysocki <rafael.j.wysocki@intel.com>,
        Tony Luck <tony.luck@intel.com>,
        Alan Cox <alan@linux.intel.com>,
        Ravi V Shankar <ravi.v.shankar@intel.com>,
        Arjan van de Ven <arjan@infradead.org>,
        linux-kernel <linux-kernel@vger.kernel.org>, x86 <x86@kernel.org>
Subject: Re: [RFC PATCH 00/16] x86/split_lock: Enable #AC exception for split
 locked accesses
Message-ID: <20180621203656.GF27616@hirez.programming.kicks-ass.net>
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On Thu, Jun 21, 2018 at 01:18:52PM -0700, Fenghua Yu wrote:
> On Thu, Jun 21, 2018 at 09:37:38PM +0200, Peter Zijlstra wrote:
> > 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.
> 
> The event name is called sq_misc.split_lock. It's been supported in perf
> already.

Thanks.

> > > 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.
> 
> This feature only triggers #AC for unaligned cache line access, not for
> other aligned (4 bytes, 8 bytes, etc). This is not explicitly said in
> ISE. I can add this info in next version of patches.

It was clear; what I'm saying it I'd like #AC to happen for any actual
unaligned LOCK access, not just across lines.