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

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Date:   Sat, 23 Jun 2018 01:03:33 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Fenghua Yu <fenghua.yu@...el.com>
cc:     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 02/16] x86/split_lock: Handle #AC exception for split
 lock in kernel mode

On Sat, 23 Jun 2018, Thomas Gleixner wrote:
> On Fri, 22 Jun 2018, Fenghua Yu wrote:
> > On Fri, Jun 22, 2018 at 01:59:44PM +0200, Thomas Gleixner wrote:
> > > Aside of that the spec says:
> > > 
> > >   31 Disable LOCK# assertion for split locked access.
> > > 
> > > Can you pretty please make sure that this bit enforces #AC enable? If 31 is
> > > ever set and such an access happens then the resulting havoc will takes
> > > ages to decode.
> > > 
> > > That bit is also mentioned in the SDM with ZERO explanation why it exists
> > > in the first place and why anyone would ever enable it and without a big
> > > fat warning about the possible consequences. Can this pretty please be
> > > fixed?
> > 
> > The bit 31 already exits on all processors. Hardware always sets its value
> > as zero after power on. It has been legacy for 20 years. It was added for
> > one customer 20 years ago. Now Intel hardware design team doesn't expect
> > anyone to set the bit.
> 
> Doesn't expect. ROTFL.
> 
> That's the most stupiest excuse for not adding a big fat warning into the
> SDM why this abomination should never be used at all.
> 
> Aside of that does the Intel hardware design team expect that this one
> customer is still depending on this nonsense and is therefore proliferating
> it forever?

Forgot to add that there are a lot of things nobody expects to be done, but
especially BIOS/SMM people have a tendency to flip random bits as they see
fit. Maybe not this one, but only for the reason that they did not notice
it yet.

Thanks,

	tglx