linux-kernel - Re: [PATCH] X86: MM: Add PAT Type write-through in combination with mtrr

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Message-ID: <20131028135847.GK4314@pd.tnic>
Date:	Mon, 28 Oct 2013 14:58:47 +0100
From:	Borislav Petkov <bp@...en8.de>
To:	Andreas Werner <wernerandy@....de>
Cc:	Ingo Molnar <mingo@...nel.org>, tglx@...utronix.de,
	mingo@...hat.com, hpa@...or.com, x86@...nel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH] X86: MM: Add PAT Type write-through in combination with
 mtrr

On Mon, Oct 28, 2013 at 01:03:58PM +0100, Andreas Werner wrote:
> > That doesn't mean you can return _PAGE_CACHE_WB for MTRR_TYPE_WRTHROUGH.
> > The correct thing to do, IMHO, would be to set the PWT bit in the PTEs
> > of those pages comprising your buffer.
> > 
> But why? the combination of PAT=WB and MTRR=WT is allowed isn´t it?
> What should i return instead of?

Well, since the pat code can't give you PWT, I'm thinking the cleaner
thing would be to do it yourself. I.e., allocate a WT buffer with
mtrr_add and then set the PWT bit of each page in that buffer.

Alternatively, your solution - albeit not that correct - is a simple one
and probably could use a good comment in the code explaining why you're
doing that if we decide to go that way.

-- 
Regards/Gruss,
    Boris.

Sent from a fat crate under my desk. Formatting is fine.
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