Skip site navigation (1)Skip section navigation (2)

FreeBSD Manual Pages

  
 
  

home | help
MEM(4)		       FreeBSD Kernel Interfaces Manual			MEM(4)

NAME
     mem, kmem -- memory files

SYNOPSIS
     device mem

DESCRIPTION
     The special file /dev/mem is an interface to the physical memory of the
     computer.	Byte offsets in	this file are interpreted as physical memory
     addresses.	 Reading and writing this file is equivalent to	reading	and
     writing memory itself.  Only offsets within the bounds of /dev/mem	are
     allowed.

     Kernel virtual memory is accessed through the interface /dev/kmem in the
     same manner as /dev/mem.  Only kernel virtual addresses that are cur-
     rently mapped to memory are allowed.

     On	ISA the	I/O memory space begins	at physical address 0x000a0000 and
     runs to 0x00100000.  The per-process data size for	the current process is
     UPAGES long, and ends at virtual address 0xf0000000.

IOCTL INTERFACE
     The MEM_EXTRACT_PADDR ioctl can be	used to	look up	the physical address
     and NUMA domain of	a given	virtual	address	in the calling process'	ad-
     dress space.  The request is described by

     struct mem_extract	{
	     uint64_t	     me_vaddr;	     /*	input */
	     uint64_t	     me_paddr;	     /*	output */
	     int	     me_domain;	     /*	output */
	     int	     me_state;	     /*	output */
     };

     The ioctl returns an error	if the address is not valid.  The information
     returned by MEM_EXTRACT_PADDR may be out of date by the time that the
     ioctl call	returns.  Specifically,	concurrent system calls, page faults,
     or	system page reclamation	activity may have unmapped the virtual page or
     replaced the backing physical page	before the ioctl call returns.	Wired
     pages, e.g., those	locked by mlock(2), will not be	reclaimed by the sys-
     tem.

     The me_state field	provides information about the state of	the virtual
     page:

     ME_STATE_INVALID
	     The virtual address is invalid.

     ME_STATE_VALID
	     The virtual address is valid but is not mapped at the time	of the
	     ioctl call.

     ME_STATE_MAPPED
	     The virtual address corresponds to	a physical page	mapping, and
	     the me_paddr and me_domain	fields are valid.

	     Several architectures allow attributes to be associated with
	     ranges of physical	memory.	 These attributes can be manipulated
	     via ioctl() calls performed on /dev/mem.  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:

	     MDF_UNCACHEABLE
		     The region	is not cached.

	     MDF_WRITECOMBINE
		     Writes to the region may be combined or performed out of
		     order.

	     MDF_WRITETHROUGH
		     Writes to the region are committed	synchronously.

	     MDF_WRITEBACK
		     Writes to the region are committed	asynchronously.

	     MDF_WRITEPROTECT
		     The region	cannot be written to.

	     Memory ranges are described by

	     struct mem_range_desc {
		     uint64_t	     mr_base;	     /*	physical base address */
		     uint64_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:

	     MDF_FIXBASE
		     The region's base address cannot be changed.

	     MDF_FIXLEN
		     The region's length cannot	be changed.

	     MDF_FIRMWARE
		     The region	is believed to have been established by	the
		     system firmware.

	     MDF_ACTIVE
		     The region	is currently active.

	     MDF_BOGUS
		     We	believe	the region to be invalid or otherwise erro-
		     neous.

	     MDF_FIXACTIVE
		     The region	cannot be disabled.

	     MDF_BUSY
		     The region	is currently owned by another process and may
		     not be altered.

	     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; a	range with the MDF_BUSY	flag may not be	removed	or up-
	     dated.

	     mo_arg[0] should be set to	MEMRANGE_SET_UPDATE to update an ex-
	     isting or establish a new range, or to MEMRANGE_SET_REMOVE	to re-
	     move a range.

RETURN VALUES
     [EOPNOTSUPP]	Memory range operations	are not	supported on this ar-
			chitecture.

     [ENXIO]		No memory range	descriptors are	available (e.g.,
			firmware has not enabled any).

     [EINVAL]		The memory range supplied as an	argument is invalid or
			overlaps another range in a fashion not	supported by
			this architecture.

     [EBUSY]		An attempt to remove or	update a range failed because
			the range is busy.

     [ENOSPC]		An attempt to create a new range failed	due to a
			shortage of hardware resources (e.g., descriptor
			slots).

     [ENOENT]		An attempt to remove a range failed because no range
			matches	the descriptor base/length supplied.

     [EPERM]		An attempt to remove a range failed because the	range
			is permanently enabled.

FILES
     /dev/mem
     /dev/kmem

SEE ALSO
     kvm(3), memcontrol(8)

HISTORY
     The mem and kmem files appeared in	Version	6 AT&T UNIX.  The ioctl	inter-
     face for memory range attributes was added	in FreeBSD 3.2.

BUGS
     Busy range	attributes are not yet managed correctly.

     This device is required for all users of kvm(3) to	operate.

FreeBSD	13.0			August 25, 2020			  FreeBSD 13.0

NAME | SYNOPSIS | DESCRIPTION | IOCTL INTERFACE | RETURN VALUES | FILES | SEE ALSO | HISTORY | BUGS

Want to link to this manual page? Use this URL:
<https://www.freebsd.org/cgi/man.cgi?query=mem&sektion=4&manpath=FreeBSD+13.0-RELEASE+and+Ports>

home | help