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

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Message-ID: <Z7ie-zG0cJFX0MOz@gmail.com>
Date: Fri, 21 Feb 2025 16:42:51 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Ard Biesheuvel <ardb@...nel.org>
Cc: Borislav Petkov <bp@...en8.de>, linux-efi@...r.kernel.org,
	linux-kernel@...r.kernel.org, x86@...nel.org, hdegoede@...hat.com
Subject: Re: [PATCH v2 0/7] x86/efi/mixed: Decouple from legacy decompressor


* Ard Biesheuvel <ardb@...nel.org> wrote:

> On Mon, 10 Feb 2025 at 18:50, Ard Biesheuvel <ardb+git@...gle.com> wrote:
> >
> > 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
> >
> 
> Unless anyone minds, I'd like to queue this up in the EFI tree.
> 
> Boris, Ingo?

Sure!

Acked-by: Ingo Molnar <mingo@...nel.org>

Thanks,

	Ingo