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

From: David Gibson
Subject: Re: Restrictions of libnet (was: Re: VW ELF loader)
Date: Wed, 5 Feb 2020 16:30:49 +1100

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.

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_!

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