linux-kernel - Re: [PATCH v2 2/6] x86, mm, pat: Change reserve

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Message-ID: <CALCETrV4DEr7tQUPCSzJMjBwgJ3-Xgcw8PFt_CCDbMoWRQ4Uug@mail.gmail.com>
Date:	Wed, 10 Sep 2014 14:27:31 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Toshi Kani <toshi.kani@...com>
Cc:	"H. Peter Anvin" <hpa@...or.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...hat.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Arnd Bergmann <arnd@...db.de>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Juergen Gross <jgross@...e.com>,
	Stefan Bader <stefan.bader@...onical.com>,
	Henrique de Moraes Holschuh <hmh@....eng.br>,
	Yigal Korman <yigal@...xistor.com>,
	Konrad Rzeszutek Wilk <konrad.wilk@...cle.com>
Subject: Re: [PATCH v2 2/6] x86, mm, pat: Change reserve_memtype() to handle WT

On Wed, Sep 10, 2014 at 2:11 PM, Toshi Kani <toshi.kani@...com> wrote:
> On Wed, 2014-09-10 at 14:06 -0700, Andy Lutomirski wrote:
>> On Wed, Sep 10, 2014 at 1:30 PM, Toshi Kani <toshi.kani@...com> wrote:
>> > On Wed, 2014-09-10 at 13:14 -0700, H. Peter Anvin wrote:
>> >> On 09/10/2014 12:30 PM, Toshi Kani wrote:
>> >> >
>> >> > When WT is unavailable due to the PAT errata, it does not fail but gets
>> >> > redirected to UC-.  Similarly, when PAT is disabled, WT gets redirected
>> >> > to UC- as well.
>> >> >
>> >>
>> >> But on pre-PAT hardware you can still do WT.
>> >
>> > Yes, if we manipulates the bits directly, but such code is no longer
>> > allowed for PAT systems.  The PAT-based kernel interfaces won't work for
>> > pre-PAT systems, and therefore requests are redirected to UC- on such
>> > systems.
>> >
>>
>> Right, the PWT bit.  Forgot about that.
>>
>> I wonder whether it would make sense to do some followup patches to
>> replace the current support for non-PAT machines with a "PAT" and
>> corresponding reverse map that exactly matches the mapping when PAT is
>> disabled.  These patches are almost there.
>
> That's possible, but the only benefit is that we can enable WT on
> pre-PAT systems, which I do not think anyone cares now...  WB & UC work
> on pre-PAT systems.  WC & WT need PAT.  I think this requirement is
> reasonable.

It might end up being a cleanup, though.  A whole bunch of
rarely-exercised if (!pat_enabled) things would go away.

--Andy
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