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

["Kirill A. Shutemov" <kirill@shutemov.name>]

On Fri, Nov 10, 2017 at 10:28:12AM +0100, Ingo Molnar wrote:
> 
> * Ingo Molnar <mingo@kernel.org> 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
> > 
> > One request: it's always going to be fragile if the _only_ thing that uses the 
> > trampoline is the 5-level paging code.
> > 
> > Could we use the trampoline in the 4-level paging case too? It's not required, but 
> > would test much of the trampoline allocation and copying machinery - and the 
> > performance cost is negligible.
> 
> Note that right now the trampoline is pointless on 4-level setups, so there's 
> nothing to copy - but we could perhaps make it meaningful. But maybe it's not a 
> good idea.

Let me see how it will play out.

> One other detail I noticed:
> 
>         /* Bound size of trampoline code */
>         .org    lvl5_trampoline_src + LVL5_TRAMPOLINE_CODE_SIZE
> 
> will this generate a build error if the trampoline code exceeds 0x40?

Yes, this is the point. Just a failsafe if trampoline code would grew too
much.

-- 
 Kirill A. Shutemov

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