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Date:   Sat, 23 Jun 2018 11:08:46 +0200 (CEST)
From:   Thomas Gleixner <tglx@linutronix.de>
To:     Fenghua Yu <fenghua.yu@intel.com>
cc:     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 02/16] x86/split_lock: Handle #AC exception for split
 lock in kernel mode
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On Fri, 22 Jun 2018, Fenghua Yu wrote:
> On Fri, Jun 22, 2018 at 01:59:44PM +0200, Thomas Gleixner wrote:
> > On Fri, 22 Jun 2018, Thomas Gleixner wrote:
> > > The whole thing is simply:
> > > 
> > > handle_ac()
> > > {
> > > 	if (user_mode(regs)) {
> > > 		 do_trap(AC, SIGBUS, ...);
> > > 	} else {
> > > 		disable_ac_on_local_cpu();
> > > 		WARN_ONCE(1);
> > > 	}
> > > }
> > > 
> > > That wants #AC enabled as early as possible so the kernel gets as much
> > > coverage as it can. If it trips in the kernel it's a bug and needs to be
> > > fixed and we can them fix ONE by ONE.
> > 
> > That said, #AC is just yet another badly defined and hastily bolted on
> > (mis)feature. This should have been:
> > 
> >   Bit A:	   Enable #AC if CPL < 3
> >   Bit B:	   Enable #AC if CPL == 3
> > 
> > But that would have been too useful and would allow sensible use of #AC
> > without creating software trainwrecks.
> >
> 
> The two bits would be ideal.

Correct. And if hardware people would not base their stuff on 'we expect'
assumptions and talk to us _before_ casting half baken features into
silicon, we would have two bits today.

Thanks,

	tglx