linux-kernel - [PATCH v2 0/7] x86/efi/mixed: Decouple from legacy decompressor

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Message-ID: <20250210174941.3251435-9-ardb+git@google.com>
Date: Mon, 10 Feb 2025 18:49:42 +0100
From: Ard Biesheuvel <ardb+git@...gle.com>
To: linux-efi@...r.kernel.org
Cc: linux-kernel@...r.kernel.org, x86@...nel.org, hdegoede@...hat.com, 
	Ard Biesheuvel <ardb@...nel.org>
Subject: [PATCH v2 0/7] x86/efi/mixed: Decouple from legacy decompressor

From: Ard Biesheuvel <ardb@...nel.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 drops substantially, but without loss of
functionality. The resulting code can operate independently from the
legacy decompressor, and is therefore moved out of arch/x86/boot/ and
into the EFI libstub/ directory.

Changes since v1 [0]:
- Create new long mode GDT that extends the firmware's 32-bit only GDT
  so that preserving/restoring data segment selectors or swapping out
  GDTs and IDTs is no longer needed at all.
- Rebase onto v6.14-rc1

[0] https://lore.kernel.org/all/20250108182218.1453754-8-ardb+git@google.com/

Ard Biesheuvel (7):
  x86/efistub: Merge PE and handover entrypoints
  x86/efi/mixed: Check CPU compatibility without relying on verify_cpu()
  x86/efi/mixed: Factor out and clean up long mode entry
  x86/efi/mixed: Set up 1:1 mapping of lower 4GiB in the stub
  x86/efi/mixed: Remove dependency on legacy startup_32 code
  x86/efi/mixed: Simplify and document thunking logic
  x86/efi/mixed: Move mixed mode startup code into libstub

 arch/x86/boot/compressed/Makefile        |   1 -
 arch/x86/boot/compressed/efi_mixed.S     | 341 --------------------
 arch/x86/boot/compressed/head_64.S       |   7 -
 drivers/firmware/efi/libstub/Makefile    |   3 +
 drivers/firmware/efi/libstub/x86-mixed.S | 253 +++++++++++++++
 drivers/firmware/efi/libstub/x86-stub.c  |  52 +--
 6 files changed, 285 insertions(+), 372 deletions(-)
 delete mode 100644 arch/x86/boot/compressed/efi_mixed.S
 create mode 100644 drivers/firmware/efi/libstub/x86-mixed.S

-- 
2.48.1.362.g079036d154-goog