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At the moment we work with a UC default MTRR type, and set three memory ranges to WB: - [0, 640 KB), - [1 MB, LowerMemorySize), - [4 GB, 4 GB + UpperMemorySize). Unfortunately, coverage for the third range can fail with a high likelihood. If the alignment of the base (ie. 4 GB) and the alignment of the size (UpperMemorySize) differ, then MtrrLib creates a series of variable MTRR entries, with power-of-two sized MTRR masks. And, it's really easy to run out of variable MTRR entries, dependent on the alignment difference. This is a problem because a Linux guest will loudly reject any high memory that is not covered my MTRR. So, let's follow the inverse pattern (loosely inspired by SeaBIOS): - flip the MTRR default type to WB, - set [0, 640 KB) to WB -- fixed MTRRs have precedence over the default type and variable MTRRs, so we can't avoid this, - set [640 KB, 1 MB) to UC -- implemented with fixed MTRRs, - set [LowerMemorySize, 4 GB) to UC -- should succeed with variable MTRRs more likely than the other scheme (due to less chaotic alignment differences). Effects of this patch can be observed by setting DEBUG_CACHE (0x00200000) in PcdDebugPrintErrorLevel. Cc: Maoming <maoming.maoming@huawei.com> Cc: Huangpeng (Peter) <peter.huangpeng@huawei.com> Cc: Wei Liu <wei.liu2@citrix.com> Contributed-under: TianoCore Contribution Agreement 1.0 Signed-off-by: Laszlo Ersek <lersek@redhat.com> Tested-by: Maoming <maoming.maoming@huawei.com> Reviewed-by: Jordan Justen <jordan.l.justen@intel.com> git-svn-id: https://svn.code.sf.net/p/edk2/code/trunk/edk2@17722 6f19259b-4bc3-4df7-8a09-765794883524