linux-kernel - Re: [PATCH 4/6] x86: efistub: Perform 4/5 level paging switch from the stub

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Message-ID: <20230426104200.drmuewhwmhh3xljh@box.shutemov.name>
Date:   Wed, 26 Apr 2023 13:42:00 +0300
From:   "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>
To:     Ard Biesheuvel <ardb@...nel.org>
Cc:     linux-efi@...r.kernel.org, linux-kernel@...r.kernel.org,
        Evgeniy Baskov <baskov@...ras.ru>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Dave Hansen <dave.hansen@...ux.intel.com>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Alexey Khoroshilov <khoroshilov@...ras.ru>,
        Peter Jones <pjones@...hat.com>,
        Gerd Hoffmann <kraxel@...hat.com>,
        Dave Young <dyoung@...hat.com>,
        Mario Limonciello <mario.limonciello@....com>,
        Kees Cook <keescook@...omium.org>,
        Tom Lendacky <thomas.lendacky@....com>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [PATCH 4/6] x86: efistub: Perform 4/5 level paging switch from
 the stub

On Mon, Apr 24, 2023 at 06:57:24PM +0200, Ard Biesheuvel wrote:
> In preparation for updating the EFI stub boot flow to avoid the bare
> metal decompressor code altogether, implement the support code for
> switching between 4 and 5 levels of paging before jumping to the kernel
> proper.

I must admit it is neat. I like it a lot.

Any chance we can share the code with the traditional decompressor?
There's not much that EFI specific here. It should be possible to isolate
it from the rest, no?


> @@ -792,6 +925,14 @@ asmlinkage unsigned long efi_main(efi_handle_t handle,
>  				(get_efi_config_table(ACPI_20_TABLE_GUID) ?:
>  				 get_efi_config_table(ACPI_TABLE_GUID));
>  
> +#ifdef CONFIG_X86_64
> +	status = efi_setup_5level_paging();
> +	if (status != EFI_SUCCESS) {
> +		efi_err("efi_setup_5level_paging() failed!\n");
> +		goto fail;
> +	}
> +#endif
> +
>  	/*
>  	 * If the kernel isn't already loaded at a suitable address,
>  	 * relocate it.
> @@ -910,6 +1051,10 @@ asmlinkage unsigned long efi_main(efi_handle_t handle,
>  		goto fail;
>  	}
>  
> +#ifdef CONFIG_X86_64
> +	efi_5level_switch();
> +#endif
> +
>  	return bzimage_addr;
>  fail:
>  	efi_err("efi_main() failed!\n");

Maybe use IS_ENABLED() + dummy efi_setup_5level_paging()/efi_5level_switch()
instead of #ifdefs?

-- 
  Kiryl Shutsemau / Kirill A. Shutemov