linux-kernel - Re: [RFC 0/3] extend kexec_file

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Date:	Wed, 13 Jul 2016 10:34:47 +0100
From:	Mark Rutland <mark.rutland@....com>
To:	Dave Young <dyoung@...hat.com>
Cc:	Arnd Bergmann <arnd@...db.de>, bhe@...hat.com,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	Vivek Goyal <vgoyal@...hat.com>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	bauerman@...ux.vnet.ibm.com, linuxppc-dev@...ts.ozlabs.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC 0/3] extend kexec_file_load system call

On Wed, Jul 13, 2016 at 10:36:14AM +0800, Dave Young wrote:
> But consider we can kexec to a different kernel and a different initrd so there
> will be use cases to pass a total different dtb as well.

It depends on what you mean by "a different kernel", and what this
implies for the DTB.

I expect future arm64 Linux kernels to function with today's DTBs, and
the existing boot protocol. The kexec_file_load syscall already has
enough information for the kernel to inject the initrd and bootargs
properties into a DTB.

In practice on x86 today, kexec_file_load only supports booting to a
Linux kernel, because the in-kernel purgatory only implements the x86
Linux boot protocol. Analagously, for arm64 I think that the first
kernel should use its internal copy of the boot DTB, with /chosen fixed
up appropriately, assuming the next kernel is an arm64 Linux image.

If booting another OS, the only parts of the DTB I would expect to
change are the properties under chosen, as everything else *should* be
OS-independent. However the other OS may have a completely different
boot protocol, might not even take a DTB, and will likely need a
compeltely different purgatory implementation. So just allowing the DTB
to be altered isn't sufficient for that case.

There might be cases where we want a different DTB, but as far as I can
tell we have nothing analagous on x86 today. If we do need this, we
should have an idea of what real case(s) were trying to solve.

Thanks,
Mark.