Re: [Xen-users] how to boot something to get a vm started

[hw <hw@xxxxxxxxxxxxxxxxxxxxxxxx>]

Am 14.07.2015 um 09:36 schrieb Ian Campbell:
> On Mon, 2015-07-13 at 19:17 +0200, hw wrote:
> > Am 13.07.2015 um 18:22 schrieb Ian Campbell:
> > > On Mon, 2015-07-13 at 18:01 +0200, hw wrote:
> > > > xc: error: Could not open kernel image (2 = No such file or directory):  
> > > > Internal error
> > > Strange. This suggests that hvmloader isn't available for some reason.
> > > What version of Xen is this? On what distro
> > It's an up-to-date Gentoo, not sure how to find out what xen version it has.
> >
> > > Do you have $prefix/lib/xen/boot/hvmloader ? If not then is it at some
> > > other path?
> > 'find / -name hvmloader' doesn't find any files, so I take it it's not
> > available.
> Indeed, and it should be, the fact that it is missing is the root cause
> of your HVM problems. You should take this up with the Gentoo ebuild
> maintainers.
>
> > Am I supopsed to expect that the VM is booting from the ISO when I
> > create it (which is what I do), or am I missing something else?
> As far as HVM guests go you are missing at least hvmloader, I can't say
> what else might be missing, but if Xen were properly installed this
> should Just Work.
It seems that there is a USE flag 'hvm' which needs to be enabled.  For 
unknown reasons, this USE flag is disabled which makes it not show up 
when the flags are queried and leaves me unable to enable it.
> If (all of) Xen were properly installed then booting an HVM guest would
> be trivial.
Well, I thought it would be relatively easy ...

> > I could also boot the whole server from an USB stick to do the
> > installation, but at least for the windoze guest, I have doubts that it
> > would run when virtualized.
> >
> > Why do they make it so impossible to install guests on xen to begin with?
> I don't know why Gentoo apparently disables HVM support by default.
Usually, you check what USE flags there are and enable just the ones you 
need and disable others you don't need.  Like there's no point in 
building emacs with X11 support when you will never use that anyway, 
there's no need to enable HVM support in case you don't need it.  That's 
perfectly fine, however, in this case, the USE flag isn't disabled in 
the usual way but in such a way that you cannot use the flag at all.
> You could install a PV guest by extracting the kernel+initrd from the
> iso and giving them as arguments in the cfg file, or by using some other
> tool (like xen-tools.org) to create a guest, or by downloading an image
> from somewhere etc.
HVM support is required in the first place, and before that doesn't 
work, I don't want to waste time with trying to get a PV guest to work. 
 Those really need to be much simpler to install.
> > What sense would it make to look for files which belong to the guest
> > which is to be installed within the file system of the host?
> There are two idioms for booting a PV guest, depending on the trust
> given to the guest admin as a host admin you may want to force a VM to
> run with a certain kernel which you have supplied. This is (or was)
> quite common in cloud environments. This avoids all sorts of sharp edges
> from a security perspective like accessing a potentially untrusted guest
> filesystem from the toolstack.
Then why not make it so that you need to explicitly specify that the 
guest shall read files residing on the host?
> In fact the Xen toolstack is not itself capable of reading guest file
> systems, it relies on tools such as pygrub to take on the task of
> extracting the bootable bits from the guest filesystem such that they
> can be booted. Like with hvmloader I don't know why Gentoo apparently
> doesn't include pygrub by default.
>
> Pvgrub is another alternative to pygrub which removes some of the
> security issues inherent in having pygrub open a guest supplied
> filesystem.
That still doesn't make sense to me.  Why can't I just boot a system 
which is capable to run as a PV guest without all this ado?  Without 
xen, I plug in the USB stick and boot from it.  Xen makes it complicated 
by not letting me boot from the stick or the ISO it was created from 
when I want to install a PV guest.
> >    When
> > specifying such files, of course I expect them to be looked for within
> > the guest.
> Then I'm afraid you haven't read the docs properly, e.g. xl.cfg(5) is
> pretty explicit about where the kernel= option looks for the file named.
Xen is extremely confusing and nothing but frustrating, and the docs on 
the wiki are a mess.  That a guest reads files residing on the host just 
like that is a good example for bad design.  Of course, the kernel does 
not reside on the host, and if I wanted to force reading a file residing 
on the host, the option for that should at least give a hint that it 
does this.  Why not use something like 'hostside_kernel' instead of 
'kernel', and/or invent a naming convention for items that are on the 
host and items that are confined within the guest?
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