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

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Message-ID: <20160714015027.GA3121@dhcp-128-65.nay.redhat.com>
Date:	Thu, 14 Jul 2016 09:50:27 +0800
From:	Dave Young <dyoung@...hat.com>
To:	Mark Rutland <mark.rutland@....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 07/13/16 at 10:34am, Mark Rutland wrote:
> 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 thought about kexec as a boot loader just like other bootloaders.
So just like a normal boot kexec should also accept external dtb.
But acutally kexec is different because it can get original dtb and
use it. So I agreed if we can not find a real use case that we have
to extend it we should keep current interface.

> 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.

Agreed.

Thanks
Dave