linux-kernel - Re: [PATCH 3/8] x86/boot/64: Add support of additional page table level during early boot

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Date:   Mon, 17 Apr 2017 12:32:25 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Andi Kleen <ak@...ux.intel.com>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        Andy Lutomirski <luto@...capital.net>,
        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>,
        "H. Peter Anvin" <hpa@...or.com>,
        Dave Hansen <dave.hansen@...el.com>,
        linux-arch@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH 3/8] x86/boot/64: Add support of additional page table
 level during early boot


* Kirill A. Shutemov <kirill@...temov.name> wrote:

> On Tue, Apr 11, 2017 at 07:09:07AM -0700, Andi Kleen wrote:
> > > I'll look closer (building proccess it's rather complicated), but my
> > > understanding is that VDSO is stand-alone binary and doesn't really links
> > > with the rest of the kernel, rather included as blob, no?
> > > 
> > > Andy, may be you have an idea?
> > 
> > There isn't any way I know of to directly link them together. The ELF 
> > format wasn't designed for that. You would need to merge blobs and then use
> > manual jump vectors, like the 16bit startup code does. It would be likely
> > complicated and ugly.
> 
> Ingo, can we proceed without coverting this assembly to C?
> 
> I'm committed to convert it to C later if we'll find reasonable solution
> to the issue.

So one way to do it would be to build it standalone as a .o, then add it not to 
the regular kernel objects link target (as you found out it's not possible to link 
32-bit and 64-bit objects), but to link it in a manual fashion, as part of 
vmlinux.bin.all-y in arch/x86/boot/compressed/Makefile.

But there would be other complications with this approach, such as we'd have to 
add a size field and there might be symbol linking problems ...

Another, pretty hacky way would be to generate a .S from the .c, then post-process 
the .S and essentially generate today's 32-bit .S from it.

Probably not worth the trouble.

Thanks,

	Ingo