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SubjectRe: [PATCH 3/4] x86/boot/compressed/64: Introduce place_trampoline()

* Kirill A. Shutemov <kirill.shutemov@linux.intel.com> wrote:

> --- a/arch/x86/boot/compressed/head_64.S
> +++ b/arch/x86/boot/compressed/head_64.S
> @@ -315,6 +315,18 @@ ENTRY(startup_64)
> * The first step is go into compatibility mode.
> */
>
> + /*
> + * Find suitable place for trampoline and populate it.
> + * The address will be stored in RCX.
> + *
> + * RSI holds real mode data and need to be preserved across
> + * a function call.
> + */
> + pushq %rsi
> + call place_trampoline
> + popq %rsi
> + movq %rax, %rcx
> +
> /* Clear additional page table */
> leaq lvl5_pgtable(%rbx), %rdi
> xorq %rax, %rax

So in the final version of this code we now have:

pushq %rsi
call need_to_enabled_l5
popq %rsi

/* If need_to_enabled_l5() returned zero, we're done here. */
cmpq $0, %rax
je lvl5

/*
* At this point we are in long mode with 4-level paging enabled,
* but we want to enable 5-level paging.
*
* The problem is that we cannot do it directly. Setting LA57 in
* long mode would trigger #GP. So we need to switch off long mode
* first.
*
* We use trampoline in lower memory to handle situation when
* bootloader put the kernel image above 4G.
*
* The first step is go into compatibility mode.
*/

/*
* Find suitable place for trampoline and populate it.
* The address will be stored in RCX.
*
* RSI holds real mode data and need to be preserved across
* a function call.
*/
pushq %rsi
call place_trampoline
popq %rsi
movq %rax, %rcx

Firstly, the 'need_to_enabled_l5' name sucks because it includes a typo, but also
because the prefix is way too generic.

Something like:

l5_paging_required()

would read a lot better - and would also provide a namespace for all L5 paging
related functions.

Secondly, couldn't this be combined into a single .c function, named accordingly:

l5_paging_prepare()

which would return true if L5 paging is available and should be enabled. In this
case the trampoline copying function would be called in C, by l5_paging_prepare().

This further reduces the amount of assembly code.

Thanks,

Ingo

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