Xen devel - Feb 2005 - RE: Building domains as a lesser user (was Re: bootloaders for domain != 0)

> On Fri, 2005-02-04 at 02:30 +0000, Ian Pratt wrote:
> > One fairly simple option is to use Linux as a domU boot loader. Boot
> > with an intrd, mount the specified filesystem, read off 
> grub.conf, display a menu over
> > the xencons, kexec the appropriate kernel.
> 
> Linux really seems like a very heavy hammer for something like this.
> Even just thinking from a resource perspective, why boot up a whole
> kernel to do nothing more than read an fs and mount another kernel.
I don''t buy the resource argument: it takes only a couple of seconds to
boot a xenU kernel to user space, and you''re about to be booting
another
kernel anyhow.
> Especially as you start thinking about things like modular fs''s,
etc,
> it''s going to be much less clean of a solution and be a
significant
> slowdown on your guest boot time.
A few seconds slow down -- nothing compared to what a BIOS normally
adds.

I don''t see why the filesystems would particularly need to be modular,
though you might do so for convenience. 
> And then, it''s yet another kernel to keep updated, etc.
I don''t see any reason to keep it up to date. Its running in a
protected
environemnt and doesn''t have any extra access that the kernel about to
be booted is going to get.
I think this approach will work well.

We already booy all of our *physical* machines using a CDROM containing
a Linux bootloader -- see xenoboot:
http://www.cl.cam.ac.uk/Research/SRG/netos/xeno/xenoboot/

Ian


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Ian Pratt wrote:
>>On Fri, 2005-02-04 at 02:30 +0000, Ian Pratt wrote:
>>
>>>One fairly simple option is to use Linux as a domU boot loader. Boot
>>>with an intrd, mount the specified filesystem, read off 
>>
>>grub.conf, display a menu over
Well I don''t think it even being grub compatible is necessary. 
I''d say
it looking a little like lilo, yaboot and the like would do just fine.

What a splendid idea!  I think this would be _very_ easy with a very 
small script out of initrd?  Or _even_ out of rc2.d!?!

	read -p ''boot: '' kernel
	kexec /boot/$kernel

-apw


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On Fri, 2005-02-04 at 09:38 +0000, Ian Pratt wrote:
> > On Fri, 2005-02-04 at 02:30 +0000, Ian Pratt wrote:
> > Especially as you start thinking about things like modular
fs''s, etc,
> > it''s going to be much less clean of a solution and be a
significant
> > slowdown on your guest boot time.
> 
> A few seconds slow down -- nothing compared to what a BIOS normally
> adds.
> 
> I don''t see why the filesystems would particularly need to be
modular,
> though you might do so for convenience. 
Because if the kernel is _different_ than every other kernel being
shipped by a distribution, then it''s a major pain.  It also ends up
giving people a lot less flexibility (because if I were to do that, for
example, it would only have ext[23] support leaving users of
reiserfs/xfs/jfs/foofs out in the cold whereas with a modular solution,
they can at least add the support for what they want).
> > And then, it''s yet another kernel to keep updated, etc.
> 
> I don''t see any reason to keep it up to date. Its running in a
protected
> environemnt and doesn''t have any extra access that the kernel
about to
> be booted is going to get.
Users don''t tend to take that answer very well ;)  The protected
environment means you can have a little bit longer to fix it, but they
have things like audit requirements, etc.  And just because it''s
running
in a protected environment doesn''t mean it''s bug-free.  Or
that it''s
going to be able to stand still as filesystem features are added, etc.
This ends up being less of a concern with minimalistic implementations
for reading filesystems like grub''s and libext2fs.

Jeremy



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On Fri, 2005-02-04 at 10:23 +0000, Andy Whitcroft wrote:
> Ian Pratt wrote:
> >>On Fri, 2005-02-04 at 02:30 +0000, Ian Pratt wrote:
> >>
> >>>One fairly simple option is to use Linux as a domU boot loader.
Boot
> >>>with an intrd, mount the specified filesystem, read off 
> >>
> >>grub.conf, display a menu over
> 
> Well I don''t think it even being grub compatible is necessary. 
I''d say
> it looking a little like lilo, yaboot and the like would do just fine.
I''d argue you want it to look like the "native" boot loader
for the
architecture.  If it doesn''t, then you have to do more special casing
to
handling updating the guest''s boot loader config.  As well as more
special casing of users'' brains telling their hands what to type ;-)

Jeremy



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> > I don''t see why the filesystems would particularly need to be
modular,
> > though you might do so for convenience.
>
> Because if the kernel is _different_ than every other kernel being
> shipped by a distribution, then it''s a major pain.  It also ends
up
> giving people a lot less flexibility (because if I were to do that, for
> example, it would only have ext[23] support leaving users of
> reiserfs/xfs/jfs/foofs out in the cold whereas with a modular solution,
> they can at least add the support for what they want).
Well we''ll have to have a special initrd to run the
"bootloader" program from
anyhow, so I guess incorporating whatever filesystems the default distro 
kernel has as modules on there won''t be particularly arduous.
> > > And then, it''s yet another kernel to keep updated, etc.
> >
> > I don''t see any reason to keep it up to date. Its running in
a protected
> > environemnt and doesn''t have any extra access that the kernel
about to
> > be booted is going to get.
>
> Users don''t tend to take that answer very well ;)  The protected
> environment means you can have a little bit longer to fix it, but they
> have things like audit requirements, etc.
OK but the kernel can be essentially airgapped from the rest of the world - 
not necessary to include network drivers, etc (or if we do, not strictly 
necessary to connect the device channels).  Security shouldn''t be the
issue,
so if it works I wouldn''t think it''d need updating regularly.

To be honest, I think whatever solution we go with is going to look a little 
messy.

Cheers,
Mark


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On Fri, 2005-02-04 at 13:27 +0000, Mark Williamson
wrote:
> > > > And then, it''s yet another kernel to keep updated,
etc.
> > >
> > > I don''t see any reason to keep it up to date. Its
running in a protected
> > > environemnt and doesn''t have any extra access that the
kernel about to
> > > be booted is going to get.
> >
> > Users don''t tend to take that answer very well ;)  The
protected
> > environment means you can have a little bit longer to fix it, but they
> > have things like audit requirements, etc.
> 
> OK but the kernel can be essentially airgapped from the rest of the world -
> not necessary to include network drivers, etc (or if we do, not strictly 
> necessary to connect the device channels).  Security shouldn''t be
the issue,
> so if it works I wouldn''t think it''d need updating
regularly.
And then you get the question "my kernel booting said it was version
x.y.z and you put out a security fix that''s x.y.z.w -- why
doesn''t my
kernel have that fix?"  Users are kind of weird like that sometimes.
And since it''s the same source, you really do want to keep the packages
in sync otherwise you have a nitemare from a packaging/distribution
perspective.

So while you might not strictly _need_ to, you''ll end up having to do
it.
> To be honest, I think whatever solution we go with is going to look a
little
> messy.
Well, it is the equivalent of a boot loader and I''ve looked at the code
to most of them... about the only thing they have in common is
"messy" :-)

Jeremy



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> I''d argue you want it to look like the "native" boot
loader for the
> architecture.  If it doesn''t, then you have to do more special
casing to
> handling updating the guest''s boot loader config.  As well as more
> special casing of users'' brains telling their hands what to type
;-)
For what i want, any valid bootloader would be fine.  So if its as dumb 
looking as lilo (yaboot) thats fine with me.

-apw


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