[arch-dev-public] Xen domU kernel - what to do?

Thomas Bächler thomas at archlinux.org
Thu Oct 29 20:59:19 EDT 2009

Okay, it is obvious that we need a kernel which can run on Xen domU for 
our servers. It would also be very convenient for those running Arch on 
hosting services that have pv-grub available.

Until now, I compiled a modified version of the kernel26 package with 
pv_ops/Xern enabled which - in theory - should also run on a real machine.

If all the external modules compile properly (which they probably do), 
we could add these options to our normal stock kernel. I haven't tried 
running it on a real machine though and tpowa said in the past the 
nvidia module failed. This has to be tested.

However, compiling Xen on a 32 bit kernel requires enabling PAE which we 
refused to do in the past for performance reasons (no idea if these are 
valid concerns) and due to the fact that we want to encourage people to 
run 64 bit if they have large amounts of memory.

Assuming all of untested things above work, what would you prefer:

1) Switch to PAE on the 32 bit kernel, ship Xen domU support with 
kernel26 on both architectures.

2) Ship Xen domU support on the 64 bit kernel, and don't on the 32 bit 
kernel (my favorite so far).

3) Leave kernel26 unchanged and create a separate kernel26-xen package 
with domU support.

The advantage of 3) is that the Xen kernel could be very slim, as almost 
all hardware drivers could be disabled. The advantage of 1) and 2) is 
the lower maintenance cost.

On a different note, I'd like to try to switch over to pv-grub on our 
servers. Right now, upgrading the kernel requires copying the kernel to 
the dom0 manually, thus dom0 access is required to do a (potentially 
security relevant) kernel update. With pv-grub, upgrading the kernel and 
rebooting would be handled just like with a normal machine, as the dom0 
only loads the pv-grub image which then runs inside the domU and can 
load the right kernel from there.
The pv-grub is only available from Xen 3.3 and later, but we have Xen 
3.2 - I compiled pv-grub from 3.4 though and guess it should work with 
the older hypervisor as well, but I haven't tested that yet.

Anyway, please post your opinion on the issues above.

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