[PATCH V2 0/3] Use efi_rts_workqueue to invoke EFI Runtime Services

[Sai Praneeth Prakhya]

From: Sai Praneeth <sai.praneeth.prakhya@xxxxxxxxx>

This patch set is an outcome of the discussion at
https://lkml.org/lkml/2017/8/21/607

Presently, efi_runtime_services() are executed by firmware in process
context. To execute efi_runtime_service(), kernel switches the page
directory from swapper_pgd to efi_pgd. However, efi_pgd doesn't have any
user space mappings. A potential issue could be, for instance, an NMI
interrupt (like perf) trying to profile some user data while in efi_pgd.

A solution for this issue could be to use kthread to run
efi_runtime_service(). When a user/kernel thread requests to execute
efi_runtime_service(), kernel off-loads this work to kthread which in
turn uses efi_pgd. Anything that tries to touch user space addresses
while in kthread is terminally broken. This patch set adds support to
the efi subsystem to handle all calls to efi_runtime_services() using a
work queue (which in turn uses kthread).

Implementation summary:
-----------------------
1. When a user/kernel thread requests to execute efi_runtime_service(),
enqueue work to a work queue, efi_rts_workqueue.
2. The caller thread waits until the work is finished because it's
dependent on the return status of efi_runtime_service() and, in specific
cases, the arguments populated by efi_runtime_service(). Some
efi_runtime_services() takes a pointer to buffer as an argument and
fills up the buffer with requested data. For instance, efi_get_variable()
and efi_get_next_variable(). Hence, the caller process cannot just post
the work and get going, it has to wait for results from firmware.

Caveat: efi_rts_workqueue to run efi_runtime_services() shouldn't be used
while in atomic, because caller thread might sleep. Presently, pstore
code doesn't use efi_rts_workqueue.

Tested using LUV (Linux UEFI Validation) for x86_64 and x86_32. Builds
fine for arm and arm64. Will appreciate the effort if someone could test
the patches on real ARM/ARM64 machines.
LUV: https://01.org/linux-uefi-validation

Thanks to Ricardo and Dan for initial reviews and suggestions. Please
feel free to pour in your comments and concerns.
Note: Patches are based on Linus's kernel v4.16-rc4

Changes from V1 to V2:
----------------------
1. Remove unnecessary include of asm/efi.h file - Fixes build error on
ia64, reported by 0-day
2. Use enum to identify efi_runtime_services()
3. Use alloc_ordered_workqueue() to create efi_rts_wq as
create_workqueue() is scheduled for depreciation.
4. Make efi_call_rts() static, as it has no callers outside
runtime-wrappers.c
5. Use BUG(), when we are unable to queue work or unable to identify
requested efi_runtime_service() - Because these two situations should
*never* happen.

Sai Praneeth (3):
  x86/efi: Call efi_delete_dummy_variable() during efi subsystem    
    initialization
  efi: Introduce efi_rts_workqueue and some infrastructure to invoke all
        efi_runtime_services()
  efi: Use efi_rts_workqueue to invoke EFI Runtime Services

 arch/x86/include/asm/efi.h              |   1 -
 arch/x86/platform/efi/efi.c             |   6 -
 drivers/firmware/efi/efi.c              |  21 +++
 drivers/firmware/efi/runtime-wrappers.c | 229 +++++++++++++++++++++++++++++---
 include/linux/efi.h                     |  23 ++++
 5 files changed, 253 insertions(+), 27 deletions(-)

Signed-off-by: Sai Praneeth Prakhya <sai.praneeth.prakhya@xxxxxxxxx>
Suggested-by: Andy Lutomirski <luto@xxxxxxxxxx>
Cc: Lee, Chun-Yi <jlee@xxxxxxxx>
Cc: Borislav Petkov <bp@xxxxxxxxx>
Cc: Tony Luck <tony.luck@xxxxxxxxx>
Cc: Will Deacon <will.deacon@xxxxxxx>
Cc: Dave Hansen <dave.hansen@xxxxxxxxx>
Cc: Mark Rutland <mark.rutland@xxxxxxx>
Cc: Bhupesh Sharma <bhsharma@xxxxxxxxxx>
Cc: Ricardo Neri <ricardo.neri@xxxxxxxxx>
Cc: Ravi Shankar <ravi.v.shankar@xxxxxxxxx>
Cc: Matt Fleming <matt@xxxxxxxxxxxxxxxxxxx>
Cc: Peter Zijlstra <peter.zijlstra@xxxxxxxxx>
Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>

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