BTW, having overlapping WC, UC regions make the end result UC. So in this

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Thanks, I noticed that later. I simply deleted the UC mapping since it was no

longer needed.

--

   -- Howard Chu

   Chief Architect, Symas Corp.  http://www.symas.com

   Director, Highland Sun        http://highlandsun.com/hyc/

   Chief Architect, OpenLDAP     http://www.openldap.org/project/

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To do this in a nicer way (and be less vulnerable to similar BIOS

funkiness) the kernel really needs full PAT support.  That should allow

WC over WB and WC over UC mappings to occur, at least if I'm

remembering the docs right...
Jesse

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PAT only really helps for device driver performance optimizations.

But if the basic WB MTRRs are wrong PAT cannot really salvage it. 
-Andi

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Look at the original bug report.  There's a bug WB region so any subsequent WC

mappings via the MTRRs won't work, so you need PAT to do WC (unless you want

to rewrite all your MTRR settings as Howard has to do now).
Seems like more and more machines are being setup this way, and until we have

PAT users will have to jump through hoops to get good performance.
Jesse

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^^^

                                          err big

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Is there any reason not to set the MTRRs to define the entire memory as

write back, and use PAT exclusively for setting cacheability?
On my home machine for instance, the BIOS uses all 8 MTRRs leaving none

for X.  I hacked it by merging a couple of MTRRs but this isn't a

generic solution.
--

error compiling committee.c: too many arguments to function
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That risks breaking SMM or BIOS code.
-Andi

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Yes.  Too bad.
--

error compiling committee.c: too many arguments to function
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BIOS should have another setup option about MTRR (continous?)

reg02: base=0x100000000 (4096MB), size=1024MB: write-back, count=1

===> if you have rev E...instead of rev F.
YH

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Thanks for the reply. Unfortunately this BIOS doesn't seem to have any other options related to the

MTRRs. I guess I'll just live with this hack.

--

   -- Howard Chu

   Chief Architect, Symas Corp.  http://www.symas.com

   Director, Highland Sun        http://highlandsun.com/hyc/

   Chief Architect, OpenLDAP     http://www.openldap.org/project/

and you will have problem...
So good way is get the ram top, and hole size...and reset the whole

set ot var mtrr.
you could refer the code in LinuxBIOS about setting that...

http://www.openbios.org/viewvc/trunk/LinuxBIOSv2/src/cpu/x86/mtrr/mtrr.c...

esp the part about CONFIG_VAR_MTRR_HOLE...
but Andi and Eric said resetting mtrr is not good... when someone from

intel try to trim the MTRR for intel CPU.
YH

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There are a couple issues with changing the MTRR configuration.

- You may not have perfect information on the cpu, the AMD revF is a good

  example.

- Code in SMM mode may actually depend on the current mtrr configuration.

- The BIOS's need to fixed to setup MTRRs properly.
So the sanest approach appears to be.

- In linux only use ram that is mapped by a write-back mtrr.

  This preserves performance and is always safe.

- If you need write-combining set it up in the page tables with PAT.
There is some difficulty there but software can always do those things

safely.
Eric

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Well the BIOS is definitely doing it wrong here. As I mentioned before, it was

setting up

	0-0x100000000 WB

	0xc0000000 - 0x100000000 UC

	0xc0000000 - 0xd0000000 WC
But the Intel Architecture Software Developer's Manual states that whenever

any variable MTRR range overlaps with an UC MTRR range, the range remains UC.

(Section 9.12.2.3). So in fact what I needed to set was

	0-2GB WB

	2-3GB WB

	3-3.25GB WC

and delete the 3-4GB UC range to get the behavior that the BIOS seems to have

been intending to set up. (Relying on the default of UC for the unspecified 
Hm. Section 9.5.1 of the doc (table 9-5) says that anything marked UC is

always UC regardless of the bits in the page table. So with the MTRR setup

that the BIOS left me with, this is still a no-go. There's no way to get the

desired effect without completely reinitializing the MTRRs.
Of course, this isn't the only problem with these Asus BIOSs...

--

   -- Howard Chu

   Chief Architect, Symas Corp.  http://www.symas.com

   Director, Highland Sun        http://highlandsun.com/hyc/

   Chief Architect, OpenLDAP     http://www.openldap.org/project/

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Which is certainly a good way to go if your hole sizes allow and you
Please look a little more closely.  WC is a flavor of UC.  There is a

specific exception that allows page tables to promote UC to WC.
I don't seem to have the same version of this document so I can't

refer you to specific sections.  But I have looked it up several

times and every time I have looked I have found the exception that
Sure.
My point was that when we try to solve the general problem rather then

a specific case there are some very practical limits on what can be

done.
Now it should be mentioned that you can go in with /proc/mtrr and fix

things manually on a specific machine.
Eric

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