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MTRR mystery (Announce)

posted by RayeR Homepage, CZ, 07.02.2012, 16:53

> Alright, now this is getting mysterious to me. Your new version seems to
> speed up all my i7 VESA lfb modes to maximum speed (between 8287 and 10070
> MB/sec!), BUT now all system memory ABOVE 2gb is Uncached! Resetting my
> system to state before MTRRLFBE.exe is run restores full caching to my
> fullrange of DOS memory available to the system (to 3.5GB)
Yes, this is because I rewrite MTRR0 that overlaped entire 4GB to 2GB. Do you have some problems caused that mem above 2GB is uncached? I guess that common dos programs will never reach this address. If this will be problem I would need add more MTRRs covering 1GB and 0.5GB or 0.25GB range upt to 3.5GB / 3.25GB. But it may happen that there will be no free MTRR. Windows and Linux should set MTRR by own way.

> Also, more strangely, if I run "mtrrlfbe lfb UC -d" on my i7 system now, it
> freezes within 0.2 second of setting the MTRRs. It is like the situation is
> reversed from your older versions of MTRRLFBE where the i7 system would
> freeze in an identical manner AFTER lfb WC was enabled!!
Hm, on my c2d and i5 it works both. I made a set of test versions to trace problem: http://rayer.ic.cz/350d/vesatest2.zip
Try all to set WC and UC and note which freezed and which not. Logs are not needed.

> It seems more apparent to me that I must manually set MTRRs somehow, but
> how? BIOS does not allow this and I do have your CPUID program, but how to
> set MTRRs with it?

try cpuid /?
this will allow you to read/write MSR. Variable MTRR0 starts at index 200h, every MTRR use two 64bit MSRs (base and mask structure). Eg. MTTR1 starta at 202h, MTRR2 at 204h and so on...
If you will clear MTRR2 (MSR index 204 and 205) on your C2D with remapping it should work. Do you need to enable remapping anyway?

> C2D with bios memory remapping before
> LFB address: D0000000h
> MTRR #0: base = 000000000h ( 0MB), mask = F00000000h (4096MB), WB, used - I fix this to 2GB
> MTRR #1: base = 100000000h (4096MB), mask = FC0000000h (1024MB), WB, used
> MTRR #2: base = 0C0000000h (3072MB), mask = FC0000000h (1024MB), UC, used - but this still overlaps LFB
> MTRR #3: base = 0BDE00000h (3038MB), mask = FFFE00000h ( 2MB), UC, used
> MTRR #4: base = 0BE000000h (3040MB), mask = FFE000000h ( 32MB), UC, used
It seems that I cannot use Page Attribute Table because it works only when paging enabled. And not all DOS programs using LFB enables paging. Even I don't know if setting will be permanent or only for program that set it. Not much info, nobody replied @osdev :(

---
DOS gives me freedom to unlimited HW access.

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