linux/Documentation/filesystems/efivarfs.rst
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   1.. SPDX-License-Identifier: GPL-2.0
   2
   3=======================================
   4efivarfs - a (U)EFI variable filesystem
   5=======================================
   6
   7The efivarfs filesystem was created to address the shortcomings of
   8using entries in sysfs to maintain EFI variables. The old sysfs EFI
   9variables code only supported variables of up to 1024 bytes. This
  10limitation existed in version 0.99 of the EFI specification, but was
  11removed before any full releases. Since variables can now be larger
  12than a single page, sysfs isn't the best interface for this.
  13
  14Variables can be created, deleted and modified with the efivarfs
  15filesystem.
  16
  17efivarfs is typically mounted like this::
  18
  19        mount -t efivarfs none /sys/firmware/efi/efivars
  20
  21Due to the presence of numerous firmware bugs where removing non-standard
  22UEFI variables causes the system firmware to fail to POST, efivarfs
  23files that are not well-known standardized variables are created
  24as immutable files.  This doesn't prevent removal - "chattr -i" will work -
  25but it does prevent this kind of failure from being accomplished
  26accidentally.
  27
  28.. warning ::
  29      When a content of an UEFI variable in /sys/firmware/efi/efivars is
  30      displayed, for example using "hexdump", pay attention that the first
  31      4 bytes of the output represent the UEFI variable attributes,
  32      in little-endian format.
  33
  34      Practically the output of each efivar is composed of:
  35
  36          +-----------------------------------+
  37          |4_bytes_of_attributes + efivar_data|
  38          +-----------------------------------+
  39
  40*See also:*
  41
  42- Documentation/admin-guide/acpi/ssdt-overlays.rst
  43- Documentation/ABI/stable/sysfs-firmware-efi-vars
  44