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MTRR(4) Device Drivers Manual (amd64) MTRR(4)

driver for CPU memory range attributes

pseudo-device mtrr 1

The mtrr device provides access to the memory range attributes supported by the MTRRs on AMD64 compatible CPUs.

Several architectures allow attributes to be associated with ranges of physical memory. These attributes can be manipulated via ioctl() calls performed on /dev/mem or /dev/xf86. Declarations and data types are to be found in <sys/memrange.h>.
The specific attributes, and number of programmable ranges may vary between architectures. The full set of supported attributes is:
The region is not cached.
Writes to the region may be combined or performed out of order.
Writes to the region are committed synchronously.
Writes to the region are committed asynchronously.
The region cannot be written to.
Memory ranges are described by
struct mem_range_desc { 
	u_int64_t	mr_base;	/∗ physical base address ∗/ 
	u_int64_t	mr_len;		/∗ physical length of region ∗/ 
	int		mr_flags;	/∗ attributes of region ∗/ 
	char		mr_owner[8]; 
In addition to the region attributes listed above, the following flags may also be set in the mr_flags field:
The region's base address cannot be changed.
The region's length cannot be changed.
The region is believed to have been established by the system firmware.
The region is currently active.
We believe the region to be invalid or otherwise erroneous.
The region cannot be disabled.
Operations are performed using
struct mem_range_op { 
	struct mem_range_desc	*mo_desc; 
	int			mo_arg[2]; 
The MEMRANGE_GET ioctl is used to retrieve current memory range attributes. If mo_arg[0] is set to 0, it will be updated with the total number of memory range descriptors. If greater than 0, the array at mo_desc will be filled with a corresponding number of descriptor structures, or the maximum, whichever is less.
The MEMRANGE_SET ioctl is used to add, alter and remove memory range attributes. A range with the MDF_FIXACTIVE flag may not be removed.
mo_arg[0] should be set to MEMRANGE_SET_UPDATE to update an existing or establish a new range, or to MEMRANGE_SET_REMOVE to remove a range.

On ioctl(2) failure, errno(2) will be set as follows:
Memory range operations are not supported on this architecture.
No memory range descriptors are available (e.g., firmware has not enabled any).
The memory range supplied as an argument is invalid or overlaps another range in a fashion not supported by this architecture.
An attempt to remove or update a range failed because the range is busy.
An attempt to create a new range failed due to a shortage of hardware resources (e.g., descriptor slots).
An attempt to remove a range failed because no range matches the descriptor base/length supplied.
An attempt to remove a range failed because the range is permanently enabled.

ioctl(2), mem(4), pctr(4), xf86(4), memconfig(8)

mtrr support was originally included in FreeBSD 3.3.
February 15, 2014 OpenBSD-current