Re: [PATCH v7 12/22] x86/decompressor: Call trampoline directly from C code

[Borislav Petkov]

On Fri, Jul 28, 2023 at 11:09:06AM +0200, Ard Biesheuvel wrote:
> Instead of returning to the asm calling code to invoke the trampoline,
> call it straight from the C code that sets the scene. That way, the
> struct return type is no longer needed for returning two values, and the
> call can be made conditional more cleanly in a subsequent patch.
> 
> This means that all callee save 64-bit registers need to be preserved
> and restored, as their contents may not survive the legacy mode switch.
> 
> Acked-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
> Signed-off-by: Ard Biesheuvel <ardb@xxxxxxxxxx>
> ---
>  arch/x86/boot/compressed/head_64.S    | 28 +++++++-----------
>  arch/x86/boot/compressed/pgtable_64.c | 30 ++++++++------------
>  2 files changed, 23 insertions(+), 35 deletions(-)
> 
> diff --git a/arch/x86/boot/compressed/head_64.S b/arch/x86/boot/compressed/head_64.S
> index 1b0c61d1b389fd37..3f38db557112c155 100644
> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -430,24 +430,12 @@ SYM_CODE_START(startup_64)
>  #endif
>  
>  	/*
> -	 * paging_prepare() sets up the trampoline and checks if we need to
> -	 * enable 5-level paging.
> -	 *
> -	 * paging_prepare() returns a two-quadword structure which lands
> -	 * into RDX:RAX:
> -	 *   - Address of the trampoline is returned in RAX.
> -	 *   - Non zero RDX means trampoline needs to enable 5-level
> -	 *     paging.
> -	 *
> +	 * set_paging_levels() updates the number of paging levels using a

I'd say here "configure paging" or so.

> diff --git a/arch/x86/boot/compressed/pgtable_64.c b/arch/x86/boot/compressed/pgtable_64.c
> index d66639c961b8eeda..649c51935fdec7ef 100644
> --- a/arch/x86/boot/compressed/pgtable_64.c
> +++ b/arch/x86/boot/compressed/pgtable_64.c
> @@ -16,11 +16,6 @@ unsigned int __section(".data") pgdir_shift = 39;
>  unsigned int __section(".data") ptrs_per_p4d = 1;
>  #endif
>  
> -struct paging_config {
> -	unsigned long trampoline_start;
> -	unsigned long l5_required;
> -};
> -
>  /* Buffer to preserve trampoline memory */
>  static char trampoline_save[TRAMPOLINE_32BIT_SIZE];
>  
> @@ -29,7 +24,7 @@ static char trampoline_save[TRAMPOLINE_32BIT_SIZE];
>   * purposes.
>   *
>   * Avoid putting the pointer into .bss as it will be cleared between
> - * paging_prepare() and extract_kernel().
> + * set_paging_levels() and extract_kernel().
>   */
>  unsigned long *trampoline_32bit __section(".data");
>  
> @@ -106,10 +101,10 @@ static unsigned long find_trampoline_placement(void)
>  	return bios_start - TRAMPOLINE_32BIT_SIZE;
>  }
>  
> -struct paging_config paging_prepare(void *rmode)
> +asmlinkage void set_paging_levels(void *rmode)

So actually "paging_prepare" or "configure_paging" is more to the point
than setting paging levels. When I see set_paging_levels() I wonder what
levels are those and I have to look at the code and go, aaah, 5-level vs
4-level.

-- 
Regards/Gruss,
    Boris.

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