linux-kernel - Re: [PATCHv6, RESEND 4/4] x86/boot/compressed/64: Handle 5-level paging boot if kernel is above 4G

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Message-ID: <CA+55aFxgsZCqJdhZJ3ztyTTFPPgkn_aH6d4ziW1g0YJKc++0+A@mail.gmail.com>
Date:   Tue, 23 Jan 2018 09:31:16 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        "the arch/x86 maintainers" <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 <linux-mm@...ck.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [PATCHv6, RESEND 4/4] x86/boot/compressed/64: Handle 5-level
 paging boot if kernel is above 4G

On Tue, Jan 23, 2018 at 9:09 AM, Kirill A. Shutemov
<kirill.shutemov@...ux.intel.com> wrote:
>
> But if the bootloader put the kernel above 4G (not sure if anybody does
> this), we would lose control as soon as paging is disabled, because the
> code becomes unreachable to the CPU.

I do wonder if we need this. Why would a bootloader ever put the data
above 4G? Does this really happen?  Wouldn't it be easier to just say
"bootloaders better put the kernel in the low 4G"?

                Linus