linux-kernel - Re: MTRR use in drivers

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Message-ID: <51C75465.9060007@zytor.com>
Date:	Sun, 23 Jun 2013 13:02:45 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Henrique de Moraes Holschuh <hmh@....eng.br>
CC:	Brice Goglin <brice.goglin@...il.com>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	David Airlie <airlied@...ux.ie>,
	dri-devel@...ts.freedesktop.org
Subject: Re: MTRR use in drivers

On 06/23/2013 12:29 PM, Henrique de Moraes Holschuh wrote:
> On Sun, 23 Jun 2013, H. Peter Anvin wrote:
>> Why do you care about performance when PAT is disabled?
> 
> It will regress already slow boxes.  We blacklist a LOT of P4s, PMs, etc and
> nobody ever took the pain to track down which ones of those actually have
> PAT+MTRR aliasing bugs.
> 
> These boxes have boards like the Radeon X300, which needs either PAT or MTRR
> to not become unusable...
> 

We're talking hardware which is now many years old, but this is causing
very serious problems on real, modern hardware.  As far as I understand
it, too, the blacklisting was precautionary (the only bug that I
personally know about is a performance bug, where WC would be
incorrectly converted to UC.)

We need a way forward here.  If it is the only way I think we would have
to sacrifice the old machines, but perhaps something can be worked out
(e.g. if PAT is disabled, fall back to MTRRs if available for ioremap_wc()).

	-hpa

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