Howard Chu <[email protected]> writes:
> Eric W. Biederman wrote:
>>> but Andi and Eric said resetting mtrr is not good... when someone from
>>> intel try to trim the MTRR for intel CPU.
>>
>> 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.
>
> 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
Ok. I do agree that is wrong, and really weird to.
> 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
> ranges.)
Which is certainly a good way to go if your hole sizes allow and you
have enough MTRRs.
>> 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.
>
> 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.
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
allows PAT to promote UC to WC.
> Of course, this isn't the only problem with these Asus BIOSs...
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
-
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to [email protected]
More majordomo info at http://vger.kernel.org/majordomo-info.html
Please read the FAQ at http://www.tux.org/lkml/
[Index of Archives]
[Kernel Newbies]
[Netfilter]
[Bugtraq]
[Photo]
[Stuff]
[Gimp]
[Yosemite News]
[MIPS Linux]
[ARM Linux]
[Linux Security]
[Linux RAID]
[Video 4 Linux]
[Linux for the blind]
[Linux Resources]
