Messages in this thread | | | Date | Fri, 10 Nov 2017 10:29:33 +0100 | From | Ingo Molnar <> | Subject | Re: [PATCH 3/4] x86/boot/compressed/64: Introduce place_trampoline() |
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* 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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