Re: [RFC PATCH 02/16] x86/split_lock: Handle #AC exception for split lock in kernel mode

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
Date: Sat, 23 Jun 2018 11:08:46 +0200 (CEST)	[thread overview]
Message-ID: <alpine.DEB.2.21.1806231106080.8650@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20180622231813.GE18979@romley-ivt3.sc.intel.com>

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