Re: [Xen-devel] Bug#759018: [PATCH RFC] Provide prebuilt grub-xen binaries for host (dom0) use

[Ian Campbell <ijc@xxxxxxxxxxxxxx>]

On Thu, 2014-09-04 at 15:15 +0100, Ian Jackson wrote:
> Colin Watson writes ("Re: Bug#759018: [PATCH RFC] Provide prebuilt grub-xen 
> binaries for host (dom0) use"):
> ...
> > There is also the question of whether the guest-side name should mention
> > GRUB.  One might argue that it shouldn't because all that matters is
> > that it uses the Multiboot protocol.  Then there is the question of who
> > gets to own the architecture names ...
> 
> The general scheme seems sound.
> 
> 
> Ian Campbell:
> 
> > > I'm not sure what the best way to promulgate the spec is -- I
> > > think a patch to add xen.git/docs/misc/pvgrub2.markdown would be
> > > sufficient (it would end up under http://xenbits.xen.org/docs/).
> 
> Since this path is in /boot/xen and contains an executable which
> expects to run in the Xen PV environment, it could also use Xen names
> for the architectures.  I don't know whether a GNU config triplet arch
> name as you suggest is easier or harder than that.

About the same. Colin suggested the GNU config triplet names in his
strawman and I couldn't think of a reason to change.

> I have a question about the spec's wording about bitness:
> 
> +It is not in general possible under Xen for a bootloader to boot a
> +kernel of a different width from itself, and this extends to
> +chainloading from a stage one. Therefore it is permissible to have
> +both `/boot/xen/pvboot-i386.elf` and `/boot/xen/pvboot-x86\_64.elf`
> +present in a guest to be used by the appropriate stage 1 (e.g. for
> +systems with 32-bit userspace and an optional 64-bit kernel).
> 
> Is it therefore expected that the host admin will be told out of band
> what bitness the guest would prefer ?  And that then the host
> toolstack will set up that bitness of guest, load its pvgrub for that
> bitness, and hope that the guest has an appropriate-bitness core image
> load in the canonical place ?

Yes. Essentially you write kernel = /path/too/pvgrub-<32bit-name> or
-<64bit-name> in your guest config. Yes, this sucks.

AIUI in cloud interfaces etc you generally have to ask for a 32- or
64-bit domain explicitly too.

> I wonder if we might, in the future, want this to be more automatic.

I suppose that Would Be Nice(tm).

I'm not sure but it might be that for a PVH guest (once grub is ported
to that) we might be able to switch bitness at runtime and avoid this
whole mess (which comes largely from the size of the p2m entries and the
difficulties in switching, which is hidden from pvh guests)

> I guess we could have a feature in the host's 64-bit pvgrub which
> would look for and load 64-bit guest pvgrub if it exists, and
> alternatively check for 32-bit guest pvgrub and if found exit
> signalling somehow to the host toolstack to restart the domain with
> the other bitness.
> 
> But what if the host has both 64- and 32-bit pvgrub but in fact only
> has one bitness of kernel ?  Signalling this back to the host by
> somehow hiding or renaming one of the bitnesses of guest pvgrub seems
> unpleasant.

You mean if all guests happen to only use one bitness? You waste a
roundtrip through a stunt domain which will do the exiting trick and
restart, or you supply the right dom0 grub to start with I guess.

> I mention this just in case there's a better way of organising this
> which might depend on a refinement to the host/guest interface.

I can't think of anything which would need to be changed in the spec now
preemptively, FWIW.

Ian.


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