Messages in this thread | | | Date | Sat, 27 Jan 2007 11:33:07 +0200 | From | Avi Kivity <> | Subject | Re: Support for i386 PATs |
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Thomas Hellström wrote: > Hi! > > Does anybody have a strong opinion against adding support for > i386 Page Attribute Tables? > > The main benefit would be that one can have write-combining memory > regions without setting up MTRRs. This will come in handy for a > device we're working with where the device driver needs to allocate the > display memory directly from system memory, and it may be difficult to > fit > the mtrr alignment constraints.
An additional benefit if that you can easily run out of mtrrs. My home machine boots with all 8 mtrrs used, leaving none for the G965 graphics. I have to compress two mtrrs into one in order to get decent performance.
> > Outline: > The PAT may be set up at boot time with fixed backwards-compatible > memory types for the different PAT entries + defines like the following: > > #define _PAGE_PAT_WB xxx > #define _PAGE_PAT_WT xxx > #define _PAGE_PAT_UC0 xxx > #define _PAGE_PAT_UC1 xxx > #define _PAGE_PAT_WC xxx > > which can be used in parallel with the old _PAGE_PWT and _PAGE_PCD bits. > > The idea is that new memory types, WC for example, will use the pat > entries > 7 downto 4, whereas 0-3 are left to boot setting to maintain backwards > compatibility. > > Issues: > 1) The _PAGE_BIT_PAT will be the same as _PAGE_PSE, and _PAGE_PROTNONE. > As I understand it, _PAGE_PROTNONE is not used when the page is > present, so this might not be an issue. > What about _PAGE_PSE?
pse pages place the pat bit in bit 12, you will need to account for that.
-- Do not meddle in the internals of kernels, for they are subtle and quick to panic.
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