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/
