linux-kernel - Re: [PATCH v2 03/16] x86/compressed: efi-mixed: move bootargs parsing out of 32-bit startup code

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Message-ID: <Yz615Qc1K284jlH9@zn.tnic>
Date:   Thu, 6 Oct 2022 13:03:01 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Michael Roth <michael.roth@....com>
Subject: Re: [PATCH v2 03/16] x86/compressed: efi-mixed: move bootargs
 parsing out of 32-bit startup code

On Wed, Sep 21, 2022 at 04:54:09PM +0200, Ard Biesheuvel wrote:
> Move the logic that chooses between the different EFI entrypoints out of
> the 32-bit boot path, and into a 64-bit helper that can perform the same
> task much more cleanly. While at it, document the mixed mode boot flow
> in a code comment.
> 
> Signed-off-by: Ard Biesheuvel <ardb@...nel.org>
> ---
>  arch/x86/boot/compressed/efi_mixed.S | 43 ++++++++++++++++++++
>  arch/x86/boot/compressed/head_64.S   | 24 ++---------
>  2 files changed, 47 insertions(+), 20 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/efi_mixed.S b/arch/x86/boot/compressed/efi_mixed.S
> index 67e7edcdfea8..77e77c3ea393 100644
> --- a/arch/x86/boot/compressed/efi_mixed.S
> +++ b/arch/x86/boot/compressed/efi_mixed.S
> @@ -22,6 +22,49 @@
>  
>  	.code64
>  	.text
> +/*
> + * When booting in 64-bit mode on 32-bit EFI firmware, startup_64_mixedmode()
> + * is the first thing that runs after switching to long mode. Depending on
> + * whether the EFI handover protocol or the compat entry point was used to
> + * enter the kernel, it will either branch to the 64-bit EFI handover
> + * entrypoint at offset 0x390 in the image, or to the 64-bit EFI PE/COFF
> + * entrypoint efi_pe_entry(). In the former case, the bootloader must provide a
> + * struct bootparams pointer as the third argument, so the presence of such a
> + * pointer is used to disambiguate.
> + *
> + *                                                             +--------------+
> + *  +------------------+     +------------+            +------>| efi_pe_entry |
> + *  | efi32_pe_entry   |---->|            |            |       +-----------+--+
> + *  +------------------+     |            |     +------+---------------+   |
> + *                           | startup_32 |---->| startup_64_mixedmode |   |
> + *  +------------------+     |            |     +------+---------------+   V
> + *  | efi32_stub_entry |---->|            |            |     +------------------+
> + *  +------------------+     +------------+            +---->| efi64_stub_entry |
> + *                                                           +-------------+----+
> + *                           +------------+     +----------+               |
> + *                           | startup_64 |<----| efi_main |<--------------+
> + *                           +------------+     +----------+
> + */

That is much appreciated.

Questions:

- is this whole handover ABI documented somewhere?

- efi32_pe_entry() is the 32-bit PE/COFF entry point? I.e., that is
called by a 32-bit EFI fw when the kernel is a PE/COFF executable?

But then Documentation/admin-guide/efi-stub.rst talks about the EFI stub
and exactly that. Hmm, so what is efi32_pe_entry() then?

> +SYM_FUNC_START(startup_64_mixedmode)

	... mixed_mode

I guess.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette