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

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Message-ID: <1431441030.24419.81.camel@misato.fc.hp.com>
Date:	Tue, 12 May 2015 08:30:30 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Borislav Petkov <bp@...en8.de>
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, 2015-05-12 at 09:28 +0200, Borislav Petkov wrote:
> On Mon, May 11, 2015 at 04:09:39PM -0600, Toshi Kani wrote:
> > There may not be any type conflict with MTRR_TYPE_INVALID.
> 
> Because...?

Because you cannot have a memory type conflict with MTRRs when MTRRs are
disabled.  mtrr_type_lookup() returns MTRR_TYPE_INVALID when MTRRs are
disabled.  This is stated in the comments of mtrr_type_lookup() and the
MTRR_TYPE_INVALID definition itself.

BIOS can disable MTRRs, or VM may choose not to implement MTRRs.  The OS
needs to handle this case as a valid config, and this is not an error
case.

> Let me guess: you cannot change this function to return a signed value
> which is the type when positive and an error when negative?

No, that is not the reason. 

> > I will change the caller to check MTRR_TYPE_INVALID, and treat it as a
> > uniform case.
> 
> That would be, of course, also wrong.

I am confused... In your previous comments, you mentioned that:

| If you want to be able to state that a type is uniform even if MTRRs
| are disabled, you need to define another retval which means exactly
| that.

There may not be type conflict when MTRRs are disabled.  There is no
point of defining a new return value.

| Or add an inline function called mtrr_enabled() and call it in the
| mtrr_type_lookup() callers.

MTRR_TYPE_INVALID means MTRRs disabled.  So, the caller checking with
this value is the same as checking with mtrr_enabled() you suggested.

Thanks,
-Toshi

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