On Fri, Nov 10, 2017 at 10:29:33AM +0100, Ingo Molnar wrote: > > * Kirill A. Shutemov <[email protected]> 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.
Makes sense. -- Kirill A. Shutemov
