efivarfs: add nostatfs mount option to skip QueryVariableInfo()

QueryVariableInfo() is an EFI runtime service that, on some firmware,
takes tens of milliseconds and runs with preemption disabled, stalling
the CPU that services it (and, on firmware that services it via SMM, all
CPUs). efivarfs_statfs() calls it (rate-limited since commit b2326338dc68
("efivarfs: Rate limit statfs() handler")) to report the variable-store
used/available capacity, so any statfs(2) -- e.g. every "df" -- can
inject that stall into unrelated latency-sensitive workloads.

Add a negatable "nostatfs" mount option: with nostatfs, statfs(2) skips
QueryVariableInfo() and reports zero used/available; with statfs it
reports the capacity as before. It defaults to nostatfs on
CONFIG_PREEMPT_RT so real-time kernels do not take the stall out of the
box, but statfs can be passed there to force reporting back on -- e.g.
for tools such as fwupd that need the efivars free space to update Secure
Boot key databases.

The option can also be toggled on a live mount via remount, so reporting
can be enabled only for the duration of a firmware update without
unmounting the boot-time efivarfs mount:

  mount -o remount,statfs   /sys/firmware/efi/efivars   # reporting on
  mount -o remount,nostatfs /sys/firmware/efi/efivars   # reporting off

A remount that does not specify statfs/nostatfs keeps the current setting.

Tested on an Intel Xeon E5-2628L v4, 6.12 kernel: a default statfs(2)
takes ~66 ms (the firmware call), versus ~11 us with nostatfs.

Suggested-by: Ard Biesheuvel <ardb@kernel.org>
Suggested-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Prashant Singh <singhpra@juniper.net>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>

0 files changed, 0 insertions, 0 deletions