linux-kernel - Re: [PATCH 4/5] x86/boot/64: Explicitly map boot

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Date:   Thu, 8 Oct 2020 11:48:36 +0200
From:   Joerg Roedel <jroedel@...e.de>
To:     Arvind Sankar <nivedita@...m.mit.edu>
Cc:     x86@...nel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 4/5] x86/boot/64: Explicitly map boot_params and command
 line

On Wed, Oct 07, 2020 at 03:53:50PM -0400, Arvind Sankar wrote:
> This is fragile, as boot_params and the command line mappings are
> required for the main kernel. If EARLY_PRINTK and RANDOMIZE_BASE are
> disabled, a QEMU/OVMF boot never accesses the command line in the
> decompressor stub, and so it never gets mapped. The main kernel accesses
> it from the identity mapping if AMD_MEM_ENCRYPT is enabled, and will
> crash.

Looked again, and I think that is wrong for boot_params, which are
touched unconditionally at the beginning of extract_kernel().

For the cmdline you are right, but one of CONFIG_ACPI,
CONFIG_RANDOMIZE_BASE, CONFIG_X86_5LEVEL or CONFIG_EARLY_PRINTK is
sufficient to have it touched during this boot stage.

Regards,

	Joerg