The efivarfs is a filesystem in the Linux kernel that enables users to create, delete, and modify (U)EFI variables. efivarfs is typically (and automatically) mounted to /sys/firmware/efi/efivars; if it needs to be mounted manually the following command can be used:
mount -t efivarfs none /sys/firmware/efi/efivars
efivarfs 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 was originally a limitation in version 0.99 of the EFI specification which was was removed before any full releases.
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 from happening accidentally.
When the content of an UEFI variable in /sys/firmware/efi/efivars is viewed, pay attention to the first 4 bytes of the output - they represent the UEFI variable attributes, in little-endian format. As a practical matter, each efivar is in the following format:
4_bytes_of_attributes + efivar_data.
CONFIG_EFIVAR_FS support needs to be enabled:
Device Drivers --->
Firmware Drivers --->
[ ] Disable EFI runtime services support by default
File systems --->
Pseudo filesystems --->
<*> EFI Variable filesystem
EFI-CSM: BIOS mode
On x86 UEFI replaced the legacy BIOS, to enable backwards compatibility during the transitional period, UEFI on x86 included a BIOS emulation, called Compatibility Support Module (CSM). When EFI-CSM is activated and in use, it will behave like a legacy BIOS, including hiding UEFI facilities from the operating system.
If this filesystem does not exist on UEFI-capable hardware it probably means that the hardware was booted in Legacy (BIOS) Mode i.e. EFI-CSM.
In most cases is a safe assumption that a computer or laptop manufactured after 2020 is a pure UEFI system that cannot be in BIOS mode; as an additional point of interest when Secure Boot is enabled EFI-CSM is automatically deactivated.
All (U)EFI functions can be disabled with the kernel parameter
efi=noruntime, or activated with
efi=runtime. A kernel booted without EFI runtime functions will not be able to alter any EFI settings and variables, including the boot configuration.