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: <20180123181335.GP7844@tassilo.jf.intel.com>
Date:   Tue, 23 Jan 2018 10:13:35 -0800
From:   Andi Kleen <ak@...ux.intel.com>
To:     "Kirill A. Shutemov" <kirill@...temov.name>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>,
        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>, 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 08:37:03PM +0300, Kirill A. Shutemov wrote:
> On Tue, Jan 23, 2018 at 09:31:16AM -0800, Linus Torvalds wrote:
> > 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"?
> 
> I don't know much about bootloaders, but do we even have such guarantee
> for in-kernel bootloader -- kexec?

There's no such guarantee, so we need it at least for kexec.

-Andi