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Date: Tue, 23 Jan 2018 10:13:35 -0800
From: Andi Kleen <ak@linux.intel.com>
To: "Kirill A. Shutemov" <kirill@shutemov.name>
Cc: Linus Torvalds <torvalds@linux-foundation.org>,
        "Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>,
        Ingo Molnar <mingo@redhat.com>,
        the arch/x86 maintainers <x86@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>, "H. Peter Anvin" <hpa@zytor.com>,
        Andy Lutomirski <luto@amacapital.net>,
        Cyrill Gorcunov <gorcunov@openvz.org>, Borislav Petkov <bp@suse.de>,
        linux-mm <linux-mm@kvack.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>
Subject: Re: [PATCHv6, RESEND 4/4] x86/boot/compressed/64: Handle 5-level
 paging boot if kernel is above 4G
Message-ID: <20180123181335.GP7844@tassilo.jf.intel.com>
References: <20180123170913.41791-1-kirill.shutemov@linux.intel.com>
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 <20180123173703.rrr7igl7xtlsawhf@node.shutemov.name>
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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@linux.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