44 lines · plain
1.. SPDX-License-Identifier: GPL-2.02 3=======================================4efivarfs - a (U)EFI variable filesystem5=======================================6 7The efivarfs filesystem was created to address the shortcomings of8using entries in sysfs to maintain EFI variables. The old sysfs EFI9variables code only supported variables of up to 1024 bytes. This10limitation existed in version 0.99 of the EFI specification, but was11removed before any full releases. Since variables can now be larger12than a single page, sysfs isn't the best interface for this.13 14Variables can be created, deleted and modified with the efivarfs15filesystem.16 17efivarfs is typically mounted like this::18 19 mount -t efivarfs none /sys/firmware/efi/efivars20 21Due to the presence of numerous firmware bugs where removing non-standard22UEFI variables causes the system firmware to fail to POST, efivarfs23files that are not well-known standardized variables are created24as immutable files. This doesn't prevent removal - "chattr -i" will work -25but it does prevent this kind of failure from being accomplished26accidentally.27 28.. warning ::29 When a content of an UEFI variable in /sys/firmware/efi/efivars is30 displayed, for example using "hexdump", pay attention that the first31 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.rst43- Documentation/ABI/removed/sysfs-firmware-efi-vars44