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
currently 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
Address Properties
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'
address 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returned by MEM_EXTRACT_PADDR may be out of date by the time that the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
system.
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.
Memory Ranges
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 erroneous.
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 updated.
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.
Live Kernel Dumps
The MEM_KERNELDUMP ioctl will initiate a kernel dump against the running
system, the contents of which will be written to a process-owned file
descriptor. The resulting dump output will be in minidump format. The
request is described by
struct mem_livedump_arg {
int fd; /* input */
int flags /* input */
uint8_t compression /* input */
};
The fd field is used to pass the file descriptor.
The flags field is currently unused and must be set to zero.
The compression field can be used to specify the desired compression to
be applied to the dump output. The supported values are defined in
<sys/kerneldump.h>; that is, KERNELDUMP_COMP_NONE, KERNELDUMP_COMP_GZIP,
or KERNELDUMP_COMP_ZSTD.
Kernel dumps taken against the running system may have inconsistent
kernel data structures due to allocation, deallocation, or modification
of memory concurrent to the dump procedure. Thus, the resulting core
dump is not guaranteed to be usable. A system under load is more likely
to produce an inconsistent result. Despite this, live kernel dumps can
be useful for offline debugging of certain types of kernel bugs, such as
deadlocks, or in inspecting a particular part of the system's state.
RETURN VALUES
MEM_EXTRACT_PADDR
The MEM_EXTRACT_PADDR ioctl always returns a value of zero.
MEMRANGE_GET/MEMRANGE_SET
[EOPNOTSUPP] Memory range operations are not supported on this
architecture.
[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.
MEM_KERNELDUMP
[EOPNOTSUPP] Kernel minidumps are not supported on this
architecture.
[EPERM] An attempt to begin the kernel dump failed because the
calling thread lacks the
[EBADF] The supplied file descriptor was invalid, or does not
have write permission.
[EBUSY] An attempt to begin the kernel dump failed because one
is already in progress.
[EINVAL] An invalid or unsupported value was specified in
flags.
[EINVAL] An invalid or unsupported compression type was
specified. PRIV_KMEM_READ privilege.
FILES
/dev/mem
/dev/kmem
SEE ALSO
kvm(3), memcontrol(8)
HISTORY
The /dev/mem file appeared in Version 1 AT&T UNIX and /dev/kmem in
Version 5 AT&T UNIX. The ioctl interface 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 15.1-STABLE-HBSD March 24, 2024 MEM(4)