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

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Date:	Mon, 16 Mar 2015 08:58:21 +0100
From:	Ingo Molnar <mingo@...nel.org>
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 v3 4/5] mtrr, x86: Clean up mtrr_type_lookup()


* Toshi Kani <toshi.kani@...com> wrote:

> MTRRs contain fixed and variable entries.  mtrr_type_lookup()
> may repeatedly call __mtrr_type_lookup() to handle a request
> that overlaps with variable entries.  However,
> __mtrr_type_lookup() also handles the fixed entries, which
> do not have to be repeated.  Therefore, this patch creates
> separate functions, mtrr_type_lookup_fixed() and
> mtrr_type_lookup_variable(), to handle the fixed and variable
> ranges respectively.
> 
> The patch also updates the function headers to clarify the
> return values and output argument.  It updates comments to
> clarify that the repeating is necessary to handle overlaps
> with the default type, since overlaps with multiple entries
> alone can be handled without such repeating.
> 
> There is no functional change in this patch.

Nice cleanup!

I also suggest adding a small table to the comments before the 
function, that lists the fixed purpose MTRRs and their address ranges 
- to make it more obvious what the magic hexadecimal constants within 
the code are doing.

> +static u8 mtrr_type_lookup_fixed(u64 start, u64 end)
> +{
> +	int idx;
> +
> +	if (start >= 0x100000)
> +		return 0xFF;

Btw., as a separate cleanup patch, we should probably also change 
'0xFF' (which is sometimes written as 0xff) to be some sufficiently 
named constant, and explain its usage somewhere?

> +	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?

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 ...

Thanks,

	Ingo
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