linux-kernel - Re: [PATCHv2, RFC] x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G

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Date:   Sat, 14 Oct 2017 18:37:57 +0300
From:   "Kirill A. Shutemov" <kirill@...temov.name>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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: [PATCHv2, RFC] x86/boot/compressed/64: Handle 5-level paging
 boot if kernel is above 4G

On Sat, Oct 14, 2017 at 09:33:53AM +0200, Ingo Molnar wrote:
> Yeah, so first most of this code should be moved from assembly to C. Any reason 
> why that cannot be done?

Well, we can move a little bit more code into C, like populating the
trampoline page, but I don't the think we can move the rest.

Switching to compatibility mode is too low-level to be written in C.

And we cannot write the trampoline code in C, as it's in 32-bit mode and
we wouldn't be able to generate it from C in a sane manner while building
64-bit kernel (we discussed this before).

-- 
 Kirill A. Shutemov