From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
        id S1752171AbdKJJ2T (ORCPT <rfc822;w@1wt.eu>);
        Fri, 10 Nov 2017 04:28:19 -0500
Received: from mail-wr0-f196.google.com ([209.85.128.196]:46574 "EHLO
        mail-wr0-f196.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
        with ESMTP id S1751223AbdKJJ2Q (ORCPT
        <rfc822;linux-kernel@vger.kernel.org>);
        Fri, 10 Nov 2017 04:28:16 -0500
X-Google-Smtp-Source: ABhQp+QxiZ2YbXHcfw40XUxZJ8zhmagG8IReBDfKSyNfsP3GFw501Y9xl0seqvDpczJPNa6kfEf3fw==
Date: Fri, 10 Nov 2017 10:28:12 +0100
From: Ingo Molnar <mingo@kernel.org>
To: "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>
Cc: Ingo Molnar <mingo@redhat.com>,
        Linus Torvalds <torvalds@linux-foundation.org>, x86@kernel.org,
        Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Cyrill Gorcunov <gorcunov@openvz.org>, Borislav Petkov <bp@suse.de>,
        Andi Kleen <ak@linux.intel.com>, linux-mm@kvack.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH 3/4] x86/boot/compressed/64: Introduce place_trampoline()
Message-ID: <20171110092812.ad2i6fj5wmdmheuf@gmail.com>
References: <20171101115503.18358-1-kirill.shutemov@linux.intel.com>
 <20171101115503.18358-4-kirill.shutemov@linux.intel.com>
 <20171110091703.7izzr7p3jkyxh7vd@gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <20171110091703.7izzr7p3jkyxh7vd@gmail.com>
User-Agent: NeoMutt/20170609 (1.8.3)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org


* 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.

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