Messages in this thread | | | Date | Tue, 24 Aug 2010 14:39:27 -0700 | From | "H. Peter Anvin" <> | Subject | Re: [RFC] - Mapping ACPI tables as CACHED |
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On 07/23/2010 05:14 PM, Henrique de Moraes Holschuh wrote: > > Well, as it was raised in this thread, ACPI tables are likely to be near RAM > regions used for IPC with the firmware or SMBIOS, and we have no idea of the > kind of crap that could happen if we enable caching on those areas. >
I'm really not sure I buy that argument -- at least not on x86: if that is the case, then when PAT is off (and we fall down to MTRR-only control) then we'd have the same failures. If we mark them cacheable and the MTRRs say uncachable, then we will *still* not cache them (since MTRR UC overrides PAT WB -- in fact "PAT off" really just means ALL the pagetables are marked WB.)
In that sense it is probably *safer* to map them WB, since the firmware if it uses page tables at all is extremely likely to have all the cache control bits at zero (meaning WB) -- and if it doesn't use page tables, they are functionally zero by default (MTRR control only.)
So I think it'd be safer to map them cacheable -- regardless of if we want to copy them to RAM or not.
-hpa
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