Xen devel - Aug 2013 - Re: [Xen-users] rump kernels running on the Xen hypervisor

On 21.8.2013 16:41, Ian Jackson wrote:
> Antti Kantee writes ("Re: [Xen-users] rump kernels running on the Xen
hypervisor"):
>> So, to answer your question, applications do not need to be explicitly
>> written to use rump kernels, but all of the interfaces used by
>> applications need to of course be provided somehow.  [...]
>
> This is all very exciting and as Ian Jackson says very similar to
> something I''ve been working on.  I started from the other end and
it
> may be that I can combine what I''ve done so far with what
you''ve done.
It would be great to find immediate synergies.  Can you be more specific 
about what you''ve been working on?
> I compiled up your example, against Xen 4.4-unstable, and I''m
afraid
> it doesn''t work for me.  Console log below.  Do you have any
> suggestions for debugging it or should I just plunge in ?
>
> Did you test this on i386 or should I rebuild as amd64 ?
I''m testing i386 dom0+domU with Xen 4.2.2.  But I think we should make 
all of them work eventually, so might as well start now.

I fixed/pushed one use-after-free which stood out.

If the above wasn''t it ... I''m not sure I can teach anything
about
debugging Xen guests on these lists.  I''ve been using simple gdbsx. 
Additionally, "l *0xEIP" in gdb has been quite effective for debugging
crashes even without gdbsx -- the rump kernel bits are quite well tested 
and everything outside of it is so simple that it''s usually easy to
just
guess what''s going wrong.  For debugging, everything is built with 
symbols, so you can dive right in.
> I think this approach will be an excellent one for stub domains such
> as qemu, proposed stub-pygrub, etc.
>
> But thinking about what you''ve done, I think we probably want to
do
> something a bit different with block and networking.
>
> Does the NetBSD VFS have
>   - tmpfs on variable-sized ramdisk
>   - romfs based on a cpio archive
> or the like ?  Producing an ffs image for something like a qemu-dm is
> going to be annoying.
You can create a FFS image with the portable makefs userspace utility 
and even edit the contents with the equally userspace fs-utils.

Though, I''m not sure what qemu-dm is or why it needs an FFS image.  For
me, this is a bit like reading the proverbial math book where they leave 
20 intermediate steps out of a proof because they''re considered
"obvious" ;)
> And often networking wants to be handled by something like SOCKS
> rather than by having an extra TCP stack in the stub domain.  The
> reason for this is that it avoids having to allocate MAC and IP
> addresses to a whole bunch of service domains; the administrator
> probably wants them to piggyback on dom0''s networking.
Ok, sounds like we shouldn''t include a full TCP/IP stack for that use 
case.  There''s something called "sockin" for rump kernels
that includes
only the sockets layer but assumes the actual networking stack is 
elsewhere.  I wrote it originally so that NFS clients in rump kernels 
could use host networking ... because configuring IP/MAC addresses for 
each NFS client wasn''t attractive.  Maybe sockin fits your use case
too?
  (I''m guessing here.  see: math book)

   - antti