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

[David Gibson]

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.

-- 
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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