linux-kernel - Re: [PATCH v4 7/7] mtrr, mm, x86: Enhance MTRR checks for KVA huge page mapping

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Message-ID: <20150512163148.GH3497@pd.tnic>
Date:	Tue, 12 May 2015 18:31:48 +0200
From:	Borislav Petkov <bp@...en8.de>
To:	Toshi Kani <toshi.kani@...com>
Cc:	akpm@...ux-foundation.org, hpa@...or.com, tglx@...utronix.de,
	mingo@...hat.com, linux-mm@...ck.org, x86@...nel.org,
	linux-kernel@...r.kernel.org, dave.hansen@...el.com,
	Elliott@...com, pebolle@...cali.nl
Subject: Re: [PATCH v4 7/7] mtrr, mm, x86: Enhance MTRR checks for KVA huge
 page mapping

On Tue, May 12, 2015 at 08:30:30AM -0600, Toshi Kani wrote:
> MTRR_TYPE_INVALID means MTRRs disabled.  So, the caller checking with
> this value is the same as checking with mtrr_enabled() you suggested.

So then you don't have to set *uniform = 1 on entry to
mtrr_type_lookup(). And change the retval test

	if ((!uniform) && (mtrr != MTRR_TYPE_WRBACK))

to
	if ((mtrr != MTRR_TYPE_INVALID) && (!uniform) && (mtrr != MTRR_TYPE_WRBACK))

You can put the MTRR_TYPE_INVALID first so that it shortcuts.

You need the distinction between MTRRs *disabled* and an MTRR region
being {non-,}uniform.

If MTRRs are disabled, uniform doesn't *mean* *anything* because it is
undefined. When MTRRs are disabled, the range is *not* covered by MTRRs
because, well, them MTRRs are disabled.

And it might be fine for *your* use case to set *uniform even when MTRRs
are disabled but it might matter in the future. So we better design it
correct from the beginning.

-- 
Regards/Gruss,
    Boris.

ECO tip #101: Trim your mails when you reply.
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