Qemu updates in raring

The raring feature freeze took effect last week. What’s been happening with qemu in the meantime?

A lot! I’ll touch on the following main changes in this post: package reorg, spice support, hugepages, uefi, and rbd support.

* package reorg

Perhaps best to begin with a bit of Ubuntu qemu packaging history. In hardy (before my time) Ubuntu shipped with separate qemu and kvm packages. This reflected the separate upstream qemu and kvm source trees. In August of 2009, upstream was already talking about merging the two trees, and Dustin Kirkland started a new qemu-kvm Ubuntu package which provided both qemu and kvm.

In 2010, a new ‘qemu-linaro’ source package was created in universe, to provide qemu with more bleeding-edge arm support from linaro. Eventually the qemu-kvm package provided the i386 and amd64 qemu-system binaries, qemu-common, and qemu-utils. All other target architecture system binaries, plus all qemu-user binaries, plus qemu-kvm-spice, came from qemu-linaro. This is clearly non-ideal from many viewpoints, and especially QA testing and bug duplication. But any reorganization would have to make sure that upgrades work seamlessly for raring-raring, quantal-raring, and future LTS-to-LTS upgrades, for the many commonly used packages (qemu-kvm, qemu on various packages, and qemu-user).

In the traditional 6-month-plus-LTS Ubuntu cycle, raring was a good time (not too close to next LTS) to try to straighten that out. It was also a good time in that upstream qemu and kvm were now very close together, and especially in that the wonderfully helpful debian qemu team which was also starting to merge debian’s qemu and qemu-kvm sources into a new qemu source tree in debian experimental.

And so, it’s done! The qemu-linaro and qemu-kvm source packages have been merged into qemu. Most arm patches from linaro are in our package, but you can still run linaro’s qemu from ppa at https://launchpad.net/~linaro-maintainers/+archive/tools/. The Ubuntu and Debian teams are working together, which should mean more stable packages in both, and combined resources in addressing bugs. Thanks especially to Michael Tokarev for helping to review the Ubuntu delta, and to infinity for more than once helping to figure out packaging issues I couldn’t have figured out on my own.

* Spice support. Spice has finally made it into main! The qemu package in main therefore finally supports spice, without having to install a separate qemu-kvm-spice package. As a simple example, if you used to do:

kvm -vga vmware -vnc :1

then you can use spice by doing:

kvm -vga qxl -spice port=5900,disable-ticketing

then connect with spicec or spicy:

spicec -h hostname -p 5900

3. Transparent hugepages. The 1.4.0 qemu release includes support for transparent hugepages. This means that when hugepages are available, qemu instances migrate some memory pages from regular to huge pages. Hugepages offer performance improvements due to (1) requiring fewer TLB entries for the same amount of memory, (2) requiring fewer lookups per page, and (3) requiring fewer page faults for nearby memory references (since each memory page is much larger).

4. Hugetlbfs mount. While transparent hugepages are convenient, if you want a particular vm to run with hugepages backing the whole VM, you will want to use dedicated hugepages. To do this, simply set KVM_HUGEPAGES to 1 in /etc/init/qemu-kvm.conf, then add an entry to /etc/sysctl.conf like:

vm.nr_hugepages = 512

(for 1G of hugepages – 512 2M pages). Make sure to leave at least around 1G of memory not dedicated to hugepages. Then add the arguments

-mem-path /run/hugepages/kvm

to your kvm command. Dedicated hugepages are not new, but the automatic mounting of the /sys/hugepages/kvm is.

6. UEFI. If you install the ovmf package, then you can run qemu with a UEFI bios (to test secureboot, for instance) by adding the ‘-bios OVMF.fd’ arguments to kvm. As was pointed out during vUDS there are some bugs to work out to make this seamless.

5. rbd. Ok this has been enabled since precise, but it’s still cool. You can use a ceph cluster to back your kvm instances (as an alternative to, say, nfs) to easily enable live migration. Just

qemu-img create -f rbd rbd:pool/vm1 10G
kvm -m 512 -drive format=rbd,file=rbd:pool/vm1 -cdrom raring.iso -boot d

See http://ceph.com/docs/master/rbd/qemu-rbd/ for more information.

So there’s what I can think of that is new in qemu this cycle. I hope you all enjoy, and if you find upgrading issues please do raise a bug.

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