linux-kernel - Re: [PATCH v3 4/5] mtrr, x86: Clean up mtrr_type

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Date:	Mon, 23 Mar 2015 13:27:13 -0600
From:	Toshi Kani <toshi.kani@...com>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	"akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"hpa@...or.com" <hpa@...or.com>,
	"tglx@...utronix.de" <tglx@...utronix.de>,
	"mingo@...hat.com" <mingo@...hat.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	"x86@...nel.org" <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"dave.hansen@...el.com" <dave.hansen@...el.com>,
	"Elliott, Robert (Server Storage)" <Elliott@...com>,
	"pebolle@...cali.nl" <pebolle@...cali.nl>
Subject: Re: [PATCH v3 4/5] mtrr, x86: Clean up mtrr_type_lookup()

On Mon, 2015-03-16 at 21:24 +0000, Kani, Toshimitsu wrote:
> > On Mar 16, 2015, at 3:58 AM, Ingo Molnar <mingo@...nel.org> wrote:
> >> * Toshi Kani <toshi.kani@...com> wrote:
 :
> 
> >> +    if (!(mtrr_state.have_fixed) ||
> >> +        !(mtrr_state.enabled & MTRR_STATE_MTRR_FIXED_ENABLED))
> > 
> > Btw., can MTRR_STATE_MTRR_FIXED_ENABLED ever be set in 
> > mtrr_state.enabled, without mtrr_state.have_fixed being set?
> 
> Yes, I believe the arch allows the fixed entries disabled
> while MTRRs are enabled.  I expect the most of systems 
> implement the fixed entries, though.

Sorry, I noticed I had mis-read your question before...

No, MTRR_STATE_MTRR_FIXED_ENABLED may not be set without
mtrr_state.have_fixed being set.  mtrr_state.have_fixed indicates if the
CPU supports MTRR fixed ranges.  So, they can be only enabled when the
CPU has ones.

> > AFAICS get_mtrr_state() will only ever fill in mtrr_state with fixed 
> > MTRRs if mtrr_state.have_fixed != 0 - but I might be mis-reading the 
> > (rather convoluted) flow of code ...
> 
> I will check the code next week.

Yes, you are right that get_mtrr_state() only fills in
mtrr_state.fixed_ranges[] when mtrr_state.have_fixed is set.  This is
because the MSRs containing the fixed ranges are only valid when this
flag is set. 

Thanks,
-Toshi



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