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282c454cd3
Clarify the usage of mtrr_lookup() in PAT code, and to make PAT code resilient to mtrr lookup problems. Specifically, pat_x_mtrr_type() is restructured to highlight, under what conditions we look for mtrr hint. pat_x_mtrr_type() uses a default type when there are any errors in mtrr lookup (still maintaining the pat consistency). And, reserve_memtype() highlights its usage ot mtrr_lookup for request type of '-1' and also defaults in a sane way on any mtrr lookup failure. pat.c looks at mtrr type of a range to get a hint on what mapping type to request when user/API: (1) hasn't specified any type (/dev/mem mapping) and we do not want to take performance hit by always mapping UC_MINUS. This will be the case for /dev/mem mappings used to map BIOS area or ACPI region which are WB'able. In this case, as long as MTRR is not WB, PAT will request UC_MINUS for such mappings. (2) user/API requests WB mapping while in reality MTRR may have UC or WC. In this case, PAT can map as WB (without checking MTRR) and still effective type will be UC or WC. But, a subsequent request to map same region as UC or WC may fail, as the region will get trackked as WB in PAT list. Looking at MTRR hint helps us to track based on effective type rather than what user requested. Again, here mtrr_lookup is only used as hint and we fallback to WB mapping (as requested by user) as default. In both cases, after using the mtrr hint, we still go through the memtype list to make sure there are no inconsistencies among multiple users. Signed-off-by: Venkatesh Pallipadi <venkatesh.pallipadi@intel.com> Signed-off-by: Suresh Siddha <suresh.b.siddha@intel.com> Tested-by: Rufus & Azrael <rufus-azrael@numericable.fr> Signed-off-by: Ingo Molnar <mingo@elte.hu> |
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