.\"     $NetBSD: x86_64_get_mtrr.2,v 1.8 2008/04/30 13:10:50 martin Exp $
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.\" Copyright (c) 2001 The NetBSD Foundation, Inc.
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.\" This code is derived from software contributed to The NetBSD Foundation
.\" by Gregory McGarry.
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.Dd November 10, 2001
.Dt X86_64_GET_MTRR 2 x86_64
.Os
.Sh NAME
.Nm x86_64_get_mtrr ,
.Nm x86_64_set_mtrr
.Nd access Memory Type Range Registers
.Sh LIBRARY
.Lb libx86_64
.Sh SYNOPSIS
.In sys/types.h
.In machine/sysarch.h
.In machine/mtrr.h
.Ft int
.Fn x86_64_get_mtrr "struct mtrr *mtrrp" "int *n"
.Ft int
.Fn x86_64_set_mtrr "struct mtrr *mtrrp" "int *n"
.Sh 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
.Xr pci 4
and
.Xr 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.
.Pp
.Fa mtrrp
is a pointer to one or more mtrr structures, as described below.
The
.Fa n
argument is a pointer to an integer containing the number of structures
pointed to by
.Fa mtrrp .
For
.Fn x86_64_set_mtrr
the integer pointed to by
.Fa n
will be updated to reflect the actual number of MTRRs successfully set.
For
.Fn x86_64_get_mtrr
no more than
.Fa n
structures will be copied out, and the integer value pointed to by
.Fa n
will be updated to reflect the actual number of valid structures retrieved.
A
.Dv NULL
argument to
.Fa mtrrp
will result in just the number of MTRRs available being returned
in the integer pointed to by
.Fa n .
.Pp
The argument
.Fa mtrrp
has the following structure:
.Bd -literal
struct mtrr {
        uint64_t base;
        uint64_t len;
        uint8_t type;
        int flags;
        pid_t owner;
};
.Ed
.Pp
The location of the mapping is described by its physical base address
.Em base
and length
.Em len .
Valid values for
.Em type
are:
.Pp
.Bl -tag -offset indent -width MTRR_TYPE_UNDEF1 -compact
.It MTRR_TYPE_UC
uncached memory
.It MTRR_TYPE_WC
use write-combining
.It MTRR_TYPE_WT
use write-through caching
.It MTRR_TYPE_WP
write-protected memory
.It MTRR_TYPE_WB
use write-back caching
.El
.Pp
Valid values for
.Em flags
are:
.Pp
.Bl -tag -offset indent -width MTRR_PRIVATE -compact
.It MTRR_PRIVATE
own range, reset the MTRR when the current process exits
.It MTRR_FIXED
use fixed range MTRR
.It MTRR_VALID
entry is valid
.El
.Pp
The
.Em owner
member is the PID of the user process which claims the mapping.
It is only valid if MTRR_PRIVATE is set in
.Em flags .
To clear/reset MTRRs, use a
.Em flags
field without MTRR_VALID set.
.Sh RETURN VALUES
Upon successful completion zero is returned, otherwise \-1 is returned
on failure, and the global variable
.Va errno
is set to indicate the error.
The integer value pointed to by
.Fa n
will contain the number of successfully processed mtrr structures
in both cases.
.Sh ERRORS
.Bl -tag -width [EINVAL]
.It Bq Er ENOSYS
The currently running kernel or CPU has no MTRR support.
.It Bq Er EINVAL
The currently running kernel has no MTRR support, or one of the mtrr
structures pointed to by
.Fa mtrrp
is invalid.
.It Bq Er EBUSY
No unused MTRRs are available.
.El
.Sh HISTORY
The
.Fn x86_64_get_mtrr
and
.Fn x86_64_set_mtrr
were derived from their i386 counterparts, which appeared in
.Nx 1.6 .