linux-kernel - Re: [RFC PATCH] ARM: Support allocating crashkernel above 4G for LPAE

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Message-ID: <ZrHgH9hFADBJrtJ6@shell.armlinux.org.uk>
Date: Tue, 6 Aug 2024 09:34:39 +0100
From: "Russell King (Oracle)" <linux@...linux.org.uk>
To: Baoquan He <bhe@...hat.com>
Cc: Jinjie Ruan <ruanjinjie@...wei.com>, akpm@...ux-foundation.org,
	linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [RFC PATCH] ARM: Support allocating crashkernel above 4G for LPAE

On Tue, Aug 06, 2024 at 04:11:30PM +0800, Baoquan He wrote:
> I am fine with it. BUT have you addressed Russell's concern, e.g how to
> test it actually?

Thanks for bringing that up.

Let me reinforce my position on this. I will _not_ be accepting a patch
that allows the crash kernel to be placed into high memory on 32-bit
ARM unless it has been thoroughly tested to prove that it can actually
work.

Right now, I don't believe it can work as placing the kernel in highmem
likely means it will be located *outside* of the lower 4GiB of physical
memory which is all that will be accessible when the MMU is turned off.
This is a pre-condition to boot a kernel - the kernel image _must_ be
located within a region of memory which is exposed to the CPU when the
MMU is turned off.

Unless it can be proven that placing the kernel in highmem means that
the kernel will be located in the lower 4GiB of physical memory space
with the MMU off, then further work on this patch is a waste of time.

-- 
RMK's Patch system: https://www.armlinux.org.uk/developer/patches/
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