Re: [RFC 0/3] extend kexec_file_load system call

From: Dave Young <dyoung@redhat.com>
To: Mark Rutland <mark.rutland@arm.com>
Cc: Arnd Bergmann <arnd@arndb.de>,
	bhe@redhat.com, kexec@lists.infradead.org,
	linux-kernel@vger.kernel.org, Vivek Goyal <vgoyal@redhat.com>,
	AKASHI Takahiro <takahiro.akashi@linaro.org>,
	"Eric W. Biederman" <ebiederm@xmission.com>,
	bauerman@linux.vnet.ibm.com, linuxppc-dev@lists.ozlabs.org,
	linux-arm-kernel@lists.infradead.org
Subject: Re: [RFC 0/3] extend kexec_file_load system call
Date: Wed, 13 Jul 2016 10:36:14 +0800	[thread overview]
Message-ID: <20160713023614.GB3222@dhcp-128-65.nay.redhat.com> (raw)
In-Reply-To: <20160712145010.GA8447@leverpostej>

On 07/12/16 at 03:50pm, Mark Rutland wrote:
> On Tue, Jul 12, 2016 at 04:24:10PM +0200, Arnd Bergmann wrote:
> > On Tuesday, July 12, 2016 10:18:11 AM CEST Vivek Goyal wrote:
> > > > 
> > > > On Open Firmware, the DT is extracted from running firmware and copied
> > > > into dynamically allocated data structures. After a kexec, the runtime
> > > > interface to the firmware is not available, so the flattened DT format
> > > > was created as a way to pass the same data in a binary blob to the new
> > > > kernel in a format that can be read from the kernel by walking the
> > > > directories in /proc/device-tree/*.
> > > 
> > > So this DT is available inside kernel and running kernel can still
> > > retrieve it and pass it to second kernel?
> > 
> > The kernel only uses the flattened DT blob at boot time and converts
> > it into the runtime data structures (struct device_node). The original
> > dtb is typically overwritten later.
> 
> On arm64 we deliberately preserved the DTB, so we can take that and
> build a new DTB from that kernel-side.
> 
> > > > - we typically ship devicetree sources for embedded machines with the
> > > >   kernel sources. As more hardware of the system gets enabled, the
> > > >   devicetree gains extra nodes and properties that describe the hardware
> > > >   more completely, so we need to use the latest DT blob to use all
> > > >   the drivers
> > > > 
> > > > - in some cases, kernels will fail to boot at all with an older version
> > > >   of the DT, or fail to use the devices that were working on the
> > > >   earlier kernel. This is usually considered a bug, but it's not rare
> > > > 
> > > > - In some cases, the kernel can update its DT at runtime, and the new
> > > >   settings are expected to be available in the new kernel too, though
> > > >   there are cases where you actually don't want the modified contents.
> > > 
> > > I am assuming that modified DT and unmodifed one both are accessible to
> > > kernel. And if user space can make decisions which modfied fields to use
> > > for new kernels and which ones not, then same can be done in kernel too?
> > 
> > The unmodified DT can typically be found on disk next to the kernel binary.
> > The option you have is to either read it from /proc/devicetree or to
> > read it from from /boot/*.dtb.
> 
> /proc/devicetree (aka /sys/firmware/devicetree) is a filesystem derived
> from the raw DTB (which is exposed at /sys/firmware/fdt).
> 
> The blob that was handed to the kernel at boot time is exposed at
> /sys/firmware/fdt.

I believe the blob can be read and passed to kexec kernel in kernel code without
the extra fd.

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. From my understanding
it is reasonable but yes I think we should think carefully about the design.

Thanks
Dave

> Thanks,
> Mark.
> 
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