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

[Kirill A. Shutemov]

On Fri, Nov 10, 2017 at 10:29:33AM +0100, Ingo Molnar wrote:
> 
> * Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx> 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