linux-kernel - Re: [PATCH] x86: mtrr cleanup for converting continuous to discrete layout v5

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Date:	Tue, 29 Apr 2008 13:04:36 -0700
From:	"Yinghai Lu" <yhlu.kernel@...il.com>
To:	"Eric W. Biederman" <ebiederm@...ssion.com>
Cc:	"Andrew Morton" <akpm@...ux-foundation.org>,
	"Ingo Molnar" <mingo@...e.hu>, "H. Peter Anvin" <hpa@...or.com>,
	"Thomas Gleixner" <tglx@...utronix.de>,
	"Gabriel C" <nix.or.die@...glemail.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"Mika Fischer" <mika.fischer@...pnet.de>
Subject: Re: [PATCH] x86: mtrr cleanup for converting continuous to discrete layout v5

On Tue, Apr 29, 2008 at 12:00 PM, Eric W. Biederman
<ebiederm@...ssion.com> wrote:
> Yinghai Lu <yhlu.kernel.send@...il.com> writes:
>
>  > some BIOS like to use continus MTRR layout, and may X driver can not add
>  > WB entries for graphical cards when 4g or more RAM installed.
>  >
>  > the patch will change MTRR to discrete.
>  >
>  > mtrr_chunk_size= could be used to have smaller continuous block to hold holes.
>  > default is 256m, could be set according to size of graphics card memory.
>  >
>  > v2: fix -1 for UC checking
>  > v3: default to disable, and need use enable_mtrr_cleanup to enable this feature
>  >     skip the var state change warning.
>  >     remove next_basek in range_to_mtrr()
>  > v4: correct warning mask.
>  > v5: CONFIG_MTRR_SANITIZER
>  >
>  > Signed-off-by: Yinghai Lu <yhlu.kernel@...il.com>
>
>  Skimming through the code it looks fairly sane.
>
>  I do think it would be good to split this patch into two pieces.
>  1) The mtrr rewriter/sanitizer/normalize.
>    All it does it should do is rewrite the MTRRs with a
>    semantically equivalent value.  This code should always be
>    safe and work on any system with MTRRs.
>
>    This works around otherwise sane bios's that simply prefer
>    to have contiguous MTRRs.
>
>    I don't see a reason why this code should be configurable.
>
>    This approach avoids earlier concerns because it starts
>    with the existing MTRR layout and not with the e820 map.
>
>  2) The mtrr_chunk_size code that rounds things off and allows
>    us to use discrete MTRRs by reducing some RAM to uncacheable.
>    Because it makes things uncacheable it has potentially bad
>    side effects on performance and thus potentially bad side
>    effects on functionality.  For areas like the SMM and ACPI
>    especially as they usually occur at the end of RAM just
>    below 4G.
>
>    The chunk size code should be configurable and default to off
>    because it has potential side effects.  A KConfig option may
>    also be appropriate.  It asks an interesting trade off question
>    do you want your BIOS to be fast or X.

(less memory + fast X) or  (more 8M RAM + slow...)

YH
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