linux-kernel - Re: PAT support for i386 and x86

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Message-ID: <20070807141042.GB952@one.firstfloor.org>
Date:	Tue, 7 Aug 2007 16:10:42 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Daniel J Blueman <daniel.blueman@...il.com>
Cc:	Andi Kleen <andi@...stfloor.org>,
	Cédric Augonnet <cedric.augonnet@...il.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>
Subject: Re: PAT support for i386 and x86_64

> What cases did you have in mind?

Mainly mapping memory which is rather tricky.

> From current Intel 64 and IA-32 Architectures Software Developer's Manual
> Volume 3A: System Programming Guide, section 10.5.2:

If you look at the detailed table it's not that simple.

> > Then there are old CPU errata that need to be handled etc.
> 
> I don't see a problem with having CONFIG_PAT depend on P4/Athlon or
> newer to avoid such workarounds for now. Better to have something
> there which works in (only) 80% of the cases, than nothing at all.

Agreed, but the patch doesn't do that.

> > You didn't solve all that at all. If it was as simple as your patch
> > we would have long done it already.
> 
> Perhaps this feature can be marked as WIP to allow this to move
> forward while corner-cases are worked out. Adding such a go/no-go

We're not going to have potentially cache corrupting features
as WIP. The last time one such bug took months to track down.

> barrier will hamper progress we do see, as it has done already for
> years.

Because it's not simple and nobody has done it properly yet.

> The PCI ordering config option years ago was a similar case, since
> various drivers didn't issue wmb()s until they were fixed up.

wmb alone is not enough for PCI ordering.

-Andi
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