I386_GET_MTRR(2) System Calls Manual (i386) I386_GET_MTRR(2)
NAME
i386_get_mtrr, i386_set_mtrraccess Memory Type Range Registers
LIBRARY
i386 Architecture Library (libi386, -li386)
SYNOPSIS
#include <sys/types.h>
#include <machine/sysarch.h>
#include <machine/mtrr.h>
int
i386_get_mtrr(struct mtrr *mtrrp, int *n);
int
i386_set_mtrr(struct mtrr *mtrrp, int *n);
DESCRIPTION
These functions provide an interface to the MTRR registers found on 686-class processors for controlling processor access to memory ranges. This is most useful for accessing devices such as video accelerators on pci(4) and agp(4) buses. For example, enabling write-combining allows bus-write transfers to be combined into a larger transfer before bursting over the bus. This can increase performance of write operations 2.5 times or more.
 
mtrrp is a pointer to one or more mtrr structures, as described below. The n argument is a pointer to an integer containing the number of structures pointed to by mtrrp. For i386_set_mtrr() the integer pointed to by n will be updated to reflect the actual number of MTRRs successfully set. For i386_get_mtrr() no more than n structures will be copied out, and the integer value pointed to by n will be updated to reflect the actual number of valid structures retrieved. A NULL argument to mtrrp will result in just the number of MTRRs available being returned in the integer pointed to by n.
 
The argument mtrrp has the following structure:
struct mtrr { uint64_t base; uint64_t len; uint8_t type; int flags; pid_t owner; };
 
The location of the mapping is described by its physical base address base and length len. Valid values for type are:
 
MTRR_TYPE_UC
uncached memory
MTRR_TYPE_WC
use write-combining
MTRR_TYPE_WT
use write-through caching
MTRR_TYPE_WP
write-protected memory
MTRR_TYPE_WB
use write-back caching
 
Valid values for flags are:
 
MTRR_PRIVATE
own range, reset the MTRR when the current process exits
MTRR_FIXED
use fixed range MTRR
MTRR_VALID
entry is valid
 
The owner member is the PID of the user process which claims the mapping. It is only valid if MTRR_PRIVATE is set in flags. To clear/reset MTRRs, use a flags field without MTRR_VALID set.
RETURN VALUES
Upon successful completion zero is returned, otherwise -1 is returned on failure, and the global variable errno is set to indicate the error. The integer value pointed to by n will contain the number of successfully processed mtrr structures in both cases.
ERRORS
[ENOSYS]
The currently running kernel or CPU has no MTRR support.
[EINVAL]
The currently running kernel has no MTRR support, or one of the mtrr structures pointed to by mtrrp is invalid.
[EBUSY]
No unused MTRRs are available.
HISTORY
The i386_get_mtrr() and i386_set_mtrr() functions appeared in NetBSD 1.6.