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Re: [Xen-users] DomU with modules?


  • To: Sven Köhler <skoehler@xxxxxx>
  • From: Adam Seering <aseering@xxxxxxxxx>
  • Date: Mon, 21 Aug 2006 18:08:24 -0400
  • Cc: xen-users@xxxxxxxxxxxxxxxxxxx
  • Delivery-date: Mon, 21 Aug 2006 15:09:06 -0700
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  • List-id: Xen user discussion <xen-users.lists.xensource.com>

Hi,

1. do you always compile seperate kernels for Dom0 and DomU? What's the
advantage?

In theory, domU kernels don't need many drivers at all, since they generally don't have physical hardware access. If you compile the kernels separately, your domU kernels can be quite small, which theoretically gives you a performance advantage. I don't know if there's another advantage to the split compiling.

I always compile a single dom0/domU kernel, though; I'm not sure how big the benefit is.

2. I would like to compile the kernel for DomU with module support. The
problem is, of course, that the Guest OS does not have access to the
module files which lie in the FS of the Dom0 somewhere. So how can i
easily access the module files from the DomU? (i don't want to copy them
to the DomU's FS - and Xen doesn't hook into the moudule-loading
mechanism, to load the files from the Dom0's FS, does it?)

Xen doesn't hook into Linux's module-loading mechanism, at least not as far as I know.

Ideally, you do want to put the modules on the domU's filesystem. If you can't do that, you could probably use an initrd image of some sort, if you only need to load modules at startup. I've never done anything unusual with an initrd; I'm sure it could be made to work, but I'd have to tinker for a while to figure out how.

If you need to load/unload modules after startup, then you do need them on some filesystem that the domU can get at. If you only need to load/unload a few modules, you could put them on the local filesystem and the others on an initrd.

3. Is there some way to access files of Dom0 within the DomUs? Is there
some other way then NFS to do this?

NFS seems to be the current standard. Obviously, any traditional network filesystem can in theory be used; just network your domU's.

A while ago, I read about an attempt to make a Xen-optimized filesystem to do exactly what you want. I don't know what's happened with this project; maybe someone else on this list knows?

4. any other tips you have?

Be prepared to be patient; it works well, but it can have a fair bit of setup and debugging time, especially if you're trying to do something unusual (I've had many hours of trouble with PCI Forwarding of on-motherboard devices and of graphics cards, for example). Test out any new Xen feature before you rely on it for your production box; if a feature was just added to xen-unstable, it may well still be a bit unstable.

Adam




Thanks,
  Sven

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