Re: Restrictions of libnet (was: Re: VW ELF loader)

[David Gibson]

On Mon, Feb 10, 2020 at 10:39:52AM +0100, Michal Suchánek wrote:
> On Mon, Feb 10, 2020 at 06:55:16PM +1100, David Gibson wrote:
> > On Wed, Feb 05, 2020 at 07:24:04AM +0100, Thomas Huth wrote:
> > > On 05/02/2020 06.30, David Gibson wrote:
> > > > On Tue, Feb 04, 2020 at 10:20:14AM +0100, Thomas Huth wrote:
> > > >> On 04/02/2020 09.54, Cornelia Huck wrote:
> > > >>> On Tue, 4 Feb 2020 07:16:46 +0100
> > > >>> Thomas Huth <address@hidden> wrote:
> > > >>>
> > > >>>> On 04/02/2020 00.26, Paolo Bonzini wrote:
> > > >>>>>
> > > >>>>>
> > > >>>>> Il mar 4 feb 2020, 00:20 Alexey Kardashevskiy <address@hidden
> > > >>>>> <mailto:address@hidden>> ha scritto:
> > > >>>>>
> > > >>>>>     Speaking seriously, what would I put into the guest?
> > > >>>>>
> > > >>>>> Only things that would be considered drivers. Ignore the partitions
> > > >>>>> issue for now so that you can just pass the device tree services to 
> > > >>>>> QEMU
> > > >>>>> with hypercalls.
> > > >>>>>
> > > >>>>>     Netboot's dhcp/tftp/ip/ipv6 client? It is going to be another 
> > > >>>>> SLOF,
> > > >>>>>     smaller but adhoc with only a couple of people knowing it.
> > > >>>>>
> > > >>>>>
> > > >>>>> You can generalize and reuse the s390 code. All you have to write 
> > > >>>>> is the
> > > >>>>> PCI scan and virtio-pci setup.  
> > > >>>>
> > > >>>> Well, for netbooting, the s390-ccw bios uses the libnet code from 
> > > >>>> SLOF,
> > > >>>> so re-using this for a slim netboot client on ppc64 would certainly 
> > > >>>> be
> > > >>>> feasible (especially since there are also already virtio drivers in 
> > > >>>> SLOF
> > > >>>> that are written in C), but I think it is not very future proof. The
> > > >>>> libnet from SLOF only supports UDP, and no TCP. So for advanced boot
> > > >>>> scenarios like booting from HTTP or even HTTPS, you need something 
> > > >>>> else
> > > >>>> (i.e. maybe grub is the better option, indeed).
> > > >>>
> > > >>> That makes me wonder what that means for s390: We're inheriting
> > > >>> libnet's limitations, but we don't have grub -- do we need to come up
> > > >>> with something different? Or improve libnet?
> > > >>
> > > >> I don't think that it makes sense to re-invent the wheel yet another
> > > >> time and write yet another TCP implementation (which is likely quite a
> > > >> bit of work, too, especially if you also want to do secure HTTPS in the
> > > >> end). So yes, in the long run (as soon as somebody seriously asks for
> > > >> HTTP booting on s390x) we need something different here.
> > > >>
> > > >> Now looking at our standard s390x bootloader zipl - this has been 
> > > >> giving
> > > >> us a headache a couple of times in the past, too (from a distro point 
> > > >> of
> > > >> view since s390x is the only major platform left that does not use 
> > > >> grub,
> > > >> but also from a s390-ccw bios point of view, see e.g.
> > > >> https://lists.gnu.org/archive/html/qemu-devel/2019-12/msg03046.html and
> > > >> related discussions).
> > > >>
> > > >> So IMHO the s390x world should move towards grub2, too. We could e.g.
> > > >> link it initially into the s390-ccw bios bios ... and if that works out
> > > >> well, later also use it as normal bootloader instead of zipl (not sure
> > > >> if that works in all cases, though, IIRC there were some size
> > > >> constraints and stuff like that).
> > > > 
> > > > petitboot would be another reasonable thing to consider here.  Since
> > > > it's Linux based, you have all the drivers you have there.  It's not
> > > > quite grub, but it does at least parse the same configuration files.
> > > > 
> > > > You do need kexec() of course, I don't know if you have that already
> > > > for s390 or not.
> > > 
> > > AFAIK we have kexec on s390. So yes, petitboot would be another option
> > > for replacing the s390-ccw bios. But when it comes to LPARs and z/VMs, I
> > > don't think it's really feasible to replace the zipl bootloader there
> > > with petitboot, so in that case grub2 still sounds like the better
> > > option to me.
> > 
> > Actually, between that and Paolo's suggestions, I thought of another
> > idea that could be helpful for both s390 and power.  Could we load
> > non-kexec() things (legacy kernels, non-Linux OSes) from Petitboot by
> > having it kexec() into a shim mini-kernel that just sets up the boot
> > environment for the other thing.
> > 
> > What I'm imagining is that petitboot loads everything that will be
> > needed for the other OS into RAM - probably as (or part of) the
> > "initrd" image.  That means the shim doesn't need to have drivers or
> > a network stack to load that in.  It just needs to construct
> > environment and jump into the real kernel.
> 
> How does that differ from what kexec normally does?
> 
> It does support the kernel format that is usually booted on the
> architecture.

It's not a question of format, but of environment.

By the time a kexec() occurs there won't be any OF client interface
available, whether or not there ever was one.  So, kexec() won't be
able to exec things which expect that to be present.  That includes
grub, AIX, and (probably) FreeBSD.

Note that while it's the same image, Linux kernels for POWER for a
long time have been able to boot in one of two ways.  One expects the
flattened tree, and an already instantiated RTAS - this method is used
directly by RTAS.  The other expects an OF client interface - it
slurps the device tree out of that and produces the flattened tree,
instantiates RTAS and then jumps to the flat tree entry point.

-- 
David Gibson                    | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au  | minimalist, thank you.  NOT _the_ _other_
                                | _way_ _around_!
http://www.ozlabs.org/~dgibson

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