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Date: Tue, 23 Jan 2018 20:37:03 +0300
From: "Kirill A. Shutemov" <kirill@shutemov.name>
Subject: Re: [PATCHv6, RESEND 4/4] x86/boot/compressed/64: Handle 5-level
 paging boot if kernel is above 4G
Message-ID: <20180123173703.rrr7igl7xtlsawhf@node.shutemov.name>
References: <20180123170913.41791-1-kirill.shutemov@linux.intel.com>
 <20180123170913.41791-5-kirill.shutemov@linux.intel.com>
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To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: "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>, Andi Kleen <ak@linux.intel.com>, linux-mm <linux-mm@kvack.org>, Linux Kernel Mailing List <linux-kernel@vger.kernel.org>

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?

-- 
 Kirill A. Shutemov

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