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