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MEM(4) 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' 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 The informa- tion returned by MEM_EXTRACT_PADDR may be out of date by the time that the ioctl call returns.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 vir- tual page or replaced the backing physical page before the ioctl call returns. Wired pages, e.g., those locked by mlock(2), will not be re- claimed 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 be- tween 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 attrib- utes. If mo_arg[0] is set to 0, it will be updated with the total num- ber of memory range descriptors. If greater than 0, the array at mo_desc will be filled with a corresponding number of descriptor struc- tures, 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 run- ning 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 sys- tem'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 be- cause 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 architec- ture. [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 spec- ified. 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 14.3 March 24, 2024 MEM(4)
NAME | SYNOPSIS | DESCRIPTION | IOCTL INTERFACE | RETURN VALUES | FILES | SEE ALSO | HISTORY | BUGS
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