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

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Message-ID: <ZrICvLH9YDHfC7MA@MiWiFi-R3L-srv>
Date: Tue, 6 Aug 2024 19:02:20 +0800
From: Baoquan He <bhe@...hat.com>
To: "Russell King (Oracle)" <linux@...linux.org.uk>
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 08/06/24 at 09:34am, Russell King (Oracle) wrote:
> 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.

Yeah, totally agree.