Re: [Xen-devel] [PROPOSAL] ARM/FDT: passing multiple binaries to a kernel

[Rob Herring <robherring2@xxxxxxxxx>]

Adding Dennis for a distro perspective.

On Wed, Sep 4, 2013 at 3:44 AM, Ian Campbell <Ian.Campbell@xxxxxxxxxx> wrote:
> On Tue, 2013-09-03 at 17:00 -0500, Rob Herring wrote:
>> On Tue, Sep 3, 2013 at 10:53 AM, Andre Przywara
>> <andre.przywara@xxxxxxxxxx> wrote:
>> > Hi,
>> >
>> > a normal Linux kernel currently supports reading the start and end address
>> > of a single binary blob via the FDT's /chosen node.
>> > This will be interpreted as the location of an initial RAM disk.
>> >
>> > The Xen hypervisor itself is a kernel, but needs up to _two_ binaries for
>> > proper operation: a Dom0 Linux kernel and it's associated initrd.
>> > On x86 this is solved via the multiboot protocol used by the Grub
>> > bootloader, which supports to pass an arbitrary number of binary modules to
>> > any kernel.
>> >
>> > Since in the ARM world we have the versatile device tree, we don't need to
>> > implement the mulitboot protocol.
>>
>> But surely there would be some advantage of reuse by using the
>> multi-boot protocol since Xen, grub, and OS tools already support it
>> for x86.
>
> Multiboot is pretty x86 specific (although MB2 has a MIPS port) and
> covers more stuff than we strictly require (e.g. on x86 it has
> requirements around which processor mode you enter in, has paging
> enabled etc).
>
>> > So I'd like to propose a new binding which denotes binary modules a kernel
>> > can use at it's own discretion.
>> > The need is triggered by the Xen hypervisor (which already uses a very
>> > similar scheme), but the approach is deliberately chosen to be as generic 
>> > as
>> > possible to allow future uses (like passing firmware blobs for devices or
>> > the like).
>> > Credits for this go to Ian Campbell, who started something very similar [1]
>> > for the Xen hypervisor. The intention of this proposal is to make this
>> > generic and publicly documented.
>>
>> Can you describe how you see the boot flow working starting with OS
>> installer writes kernel, initrd, xen and ??? to disk.
>
> Kernel and initrd are written to /boot in the usual way (probably from
> kernel.deb or whatever). Xen would also normally come from a distro
> package (also in /boot).
>
>> How does the bootloader know what to load?
>
> It's in the bootloader config, e.g. boot.scr or grub.cfg, which are
> either hand written or produced by the distros tooling.
>
> grub on ARM could consume the same stanzas as are used by grub on x86 to
> boot Xen (which are produced by update-grub):
>         echo    'Loading Xen 4.1-amd64 ...'
>         multiboot       /xen-4.1-amd64.gz placeholder
>         echo    'Loading Linux 3.10-2-amd64 ...'
>         module  /vmlinuz-3.10-2-amd64 placeholder root=/dev/mapper/disks-root 
> ro resume=/dev/mapper/disks-swap quiet
>         echo    'Loading initial ramdisk ...'
>         module  /initrd.img-3.10-2-amd64
>
> Since there is no multiboot on ARM (and never will be) this is safe.
>
> If multiboot ever does come to ARM it will necessarily be multiboot2
> which uses a different keyword.

Right, this is just a text file with a list of binaries. It is not
really the multiboot spec. There is no reason for this part to be
different for grub on ARM. There is a big advantage to reusing the
distro side tooling. If there isn't really much reuse on the
bootloader side, then I'm fine with a different bootloader to Xen
interface. I would like to hear that from folks working on grub
though.

> For u-boot Andre has proposed some syntactic sugar over the "fdt"
> command to make boot.scr more trivial to use. We would of course need to
> implement support for using it in the relevant distro tools (but they
> tend to be very distro/machine specific already, e.g. Debian's
> flash-kernel)

And being machine specific is a PITA. flash-kernel is certainly not
something we want to expand on. There is not much love for boot.scr
either. There is work to address what are not really machine
differences, but largely vendor u-boot differences:

http://www.mail-archive.com/u-boot@xxxxxxxxxxxxx/msg119025.html

One option for u-boot which already supports syslinux style menu files
is to adopt the syslinux multiboot parsing support:

http://www.syslinux.org/wiki/index.php/Doc/mboot


We need to back-up and consider what this looks like in the end for
all the pieces and get input from folks on grub, UEFI, and armv8. The
UEFI answer may be this is a grub problem. For armv8, this proposal
does match up well as the kernel boot interface for v8 is DT. Despite
some claims, ACPI will not completely replace DT because of this.

Rob

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