linux-kernel - Re: [PATCH 3/4] x86/boot/compressed/64: Introduce place

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Date:   Fri, 10 Nov 2017 10:28:12 +0100
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Andy Lutomirski <luto@...capital.net>,
        Cyrill Gorcunov <gorcunov@...nvz.org>,
        Borislav Petkov <bp@...e.de>, Andi Kleen <ak@...ux.intel.com>,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/4] x86/boot/compressed/64: Introduce place_trampoline()


* Ingo Molnar <mingo@...nel.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.

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?

Thanks,

	Ingo