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[0/6] x86/efi/mixed: Decouple from legacy decompressor

Message ID 20250108182218.1453754-8-ardb+git@google.com
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Series x86/efi/mixed: Decouple from legacy decompressor | expand

Message

Ard Biesheuvel Jan. 8, 2025, 6:22 p.m. UTC 
From: Ard Biesheuvel <ardb@kernel.org>

Since commit

  a1b87d54f4e4 ("x86/efistub: Avoid legacy decompressor when doing EFI boot")

booting via the EFI stub no longer relies on the legacy decompressor,
and instead, the kernel proper is decompressed by code executing in the
context of the EFI boot services, and subsequently invoked directly.

The only remaining dependency is the EFI mixed mode startup code, which
makes a detour via the legacy decompressor's 32-bit entrypoint, in order
to obtain a 1:1 mapping of memory, which is a prerequisite for 64-bit
execution on x86.

This detour requires some fiddly setup on the part of the mixed mode
startup code, which has to stash the firmware stack pointer and boot
arguments in memory, and create a fake struct boot_params to trick the
code in startup_32 to behave as intended.

This dependency also impedes reuse of the EFI stub code in other
contexts, such as generic EFI zboot, which will reuse the EFI stub but
not the legacy decompressor.

So remove this dependency, by replacing this detour with a minimal
reimplementation of the 1:1 mapping code. With some further cleanup
applied on top, the line count barely changes, but the resulting code
can operate independently from the legacy decompressor, and is therefore
moved out of arch/x86/boot/compressed and into the EFI libstub/
directory.

Ard Biesheuvel (6):
  x86/efi/mixed: Check CPU compatibility without relying on verify_cpu()
  x86/efi/mixed: Remove dependency on legacy startup_32 code
  x86/efi/mixed: Don't bother preserving 64-bit mode segment selectors
  x86/efi/mixed: Simplify and document thunking logic
  x86/efi/mixed: Reduce padding by moving some code around
  x86/efi/mixed: Move mixed mode startup code into libstub

 arch/x86/boot/compressed/Makefile                                                |   1 -
 arch/x86/boot/compressed/head_64.S                                               |   7 -
 drivers/firmware/efi/libstub/Makefile                                            |   3 +
 arch/x86/boot/compressed/efi_mixed.S => drivers/firmware/efi/libstub/x86-mixed.S | 320 ++++++++++----------
 4 files changed, 169 insertions(+), 162 deletions(-)
 rename arch/x86/boot/compressed/efi_mixed.S => drivers/firmware/efi/libstub/x86-mixed.S (54%)