| .. SPDX-License-Identifier: GPL-2.0 |
| |
| ======================================= |
| efivarfs - a (U)EFI variable filesystem |
| ======================================= |
| |
| The efivarfs filesystem was created to address the shortcomings of |
| using entries in sysfs to maintain EFI variables. The old sysfs EFI |
| variables code only supported variables of up to 1024 bytes. This |
| limitation existed in version 0.99 of the EFI specification, but was |
| removed before any full releases. Since variables can now be larger |
| than a single page, sysfs isn't the best interface for this. |
| |
| Variables can be created, deleted and modified with the efivarfs |
| filesystem. |
| |
| efivarfs is typically mounted like this:: |
| |
| mount -t efivarfs none /sys/firmware/efi/efivars |
| |
| Due to the presence of numerous firmware bugs where removing non-standard |
| UEFI variables causes the system firmware to fail to POST, efivarfs |
| files that are not well-known standardized variables are created |
| as immutable files. This doesn't prevent removal - "chattr -i" will work - |
| but it does prevent this kind of failure from being accomplished |
| accidentally. |
| |
| .. warning :: |
| When a content of an UEFI variable in /sys/firmware/efi/efivars is |
| displayed, for example using "hexdump", pay attention that the first |
| 4 bytes of the output represent the UEFI variable attributes, |
| in little-endian format. |
| |
| Practically the output of each efivar is composed of: |
| |
| +-----------------------------------+ |
| |4_bytes_of_attributes + efivar_data| |
| +-----------------------------------+ |
| |
| *See also:* |
| |
| - Documentation/admin-guide/acpi/ssdt-overlays.rst |
| - Documentation/ABI/stable/sysfs-firmware-efi-vars |
