linux-kernel - Re: [PATCH 21/26] x86/mm: add support of additional page table level during early boot

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Date:   Wed, 5 Apr 2017 14:36:24 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, x86@...nel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Arnd Bergmann <arnd@...db.de>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <ak@...ux.intel.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Andy Lutomirski <luto@...capital.net>,
        Michal Hocko <mhocko@...e.com>, linux-arch@...r.kernel.org,
        linux-mm@...ck.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 21/26] x86/mm: add support of additional page table level
 during early boot

On Mon, Mar 13, 2017 at 08:18:10AM +0100, Ingo Molnar wrote:
> 
> * Kirill A. Shutemov <kirill.shutemov@...ux.intel.com> wrote:
> 
> > This patch adds support for 5-level paging during early boot.
> > It generalizes boot for 4- and 5-level paging on 64-bit systems with
> > compile-time switch between them.
> > 
> > Signed-off-by: Kirill A. Shutemov <kirill.shutemov@...ux.intel.com>
> > ---
> >  arch/x86/boot/compressed/head_64.S          | 23 +++++++++--
> >  arch/x86/include/asm/pgtable.h              |  2 +-
> >  arch/x86/include/asm/pgtable_64.h           |  6 ++-
> >  arch/x86/include/uapi/asm/processor-flags.h |  2 +
> >  arch/x86/kernel/espfix_64.c                 |  2 +-
> >  arch/x86/kernel/head64.c                    | 40 +++++++++++++-----
> >  arch/x86/kernel/head_64.S                   | 63 +++++++++++++++++++++--------
> 
> Ok, here I'd like to have a C version instead of further complicating an already 
> complex assembly version...

Just head up: I work on this.

It's great deal of frustration (I can't really read assembly), but I'm
slowly moving forward.

Most of logic in startup_64 in arch/x86/kernel/head_64.S is converted
to C. Dealing with secondary_startup_64 now.

Not sure if it's possible to convert code in
arch/x86/boot/compressed/head_64.S to C.

Assembly code there is in 32-bit mode, but if we move it to C it will
compiled as 64-bit. I've tried to put it in separate translation unit and
compile with -m32, but then link phase breaks as object files have
different types.

Any suggestion how I can get out of the situation?

-- 
 Kirill A. Shutemov