MTRR stands for Memory Type range registers which is a new way of
partitioning and managing memory resources in your system. This
feature had been placed in processors due to memory sizes becoming
bigger and bigger. There had to be a way that the memory could be
allocated addressed and used more effectively.
A main feature of the MTRR is to map locations of PCI or AGP
components in your system so that software, drivers could access them
quickly and efficiently.
(from Gentoo Wiki)
After reading Solving linux MTRR problems it appears that the error you see is reported when mtrr sanitizer is unable to choose from several options of memory layout. It should print a list of possible options next to the error message. To make the message to go away, you need to specify something like
enable_mtrr_cleanup mtrr_spare_reg_nr=1 mtrr_gran_size=32M mtrr_chunk_size=128M
in the kernal boot parameters (where the actual values come from one of the options proposed by the sanitizer).
The author of the article claims that
Now 24MiB of ram are gone, but my 3d workloads, which are basically
webgl content and the hidden flight simulator inside google earth, run
both at significant better framerates.
which makes me very interested... need to check
dmesg on all my machines.