Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
Date:   Fri, 22 Jun 2018 10:11:46 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Fenghua Yu <fenghua.yu@intel.com>
Cc:     Dave Hansen <dave.hansen@intel.com>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@elte.hu>,
        "H. Peter Anvin" <hpa@linux.intel.com>,
        Ashok Raj <ashok.raj@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: <20180622081146.GG2494@hirez.programming.kicks-ass.net>
References: <1527435965-202085-1-git-send-email-fenghua.yu@intel.com>
 <20180621193738.GA13636@worktop.programming.kicks-ass.net>
 <20180621201851.GC114883@romley-ivt3.sc.intel.com>
 <alpine.DEB.2.21.1806212223480.1591@nanos.tec.linutronix.de>
 <20180621220003.GD114883@romley-ivt3.sc.intel.com>
 <20180621220844.GB2494@hirez.programming.kicks-ass.net>
 <6fbff2df-96c9-db51-371b-a23d1a84d070@intel.com>
 <20180621224203.GA18979@romley-ivt3.sc.intel.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20180621224203.GA18979@romley-ivt3.sc.intel.com>
User-Agent: Mutt/1.10.0 (2018-05-17)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Thu, Jun 21, 2018 at 03:42:03PM -0700, Fenghua Yu wrote:
> On Thu, Jun 21, 2018 at 03:18:19PM -0700, Dave Hansen wrote:
> > On 06/21/2018 03:08 PM, Peter Zijlstra wrote:
> > > On Thu, Jun 21, 2018 at 03:00:03PM -0700, Fenghua Yu wrote:
> > >> The control knob for kernel is to choose re-executing the faulting
> > >> instruction (default) or kernel panic. Kernel panic may be useful in hard
> > >> real time which has less tolerant to bad performance.
> > > The kernel should never trigger this, ever. Any #AC is a plain bug and
> > > should be fixed. So unconditional WARN+disable is suffient.
> > 
> > It sounds like we need to start with that functionality and then if
> > Fenghua wants to do more, we have add-on patches that can be evaluated
> > by themselves.
> 
> For split lock in kernel, by default, I just do warning, disable feature
> in #AC handler, continue to execute the faulting instruction, and re-enable
> #AC for split lock after a delay. This is the default behavior in the
> current patches.
> 
> By "WAR+disable", you mean I just do this default behavior and remove the
> control knob?

No, remove all that broken delay crap, remove the knob. Basically remove
everything.

The handler really should be:

do_handle_ac_trap()
{
	if (user_mode(regs)) {
		send_signal(SIGBUS);
		return;
	}

	WARN_ON(1);
	disable_ac_on_all_cpus();
}

done. No broken delay code, no nothing.