Filed under misc

Git Workflow

Havoc's recent post on git was interesting because it shows how frustrating git can be if you try and treat it as "just another CVS". From that perspective, git just seems like it's just some bizarre way for kernel hackers to torture those who just want to get work done.

I turned that corner with git when I learned about "git-rebase -i" and came to the startling realisation that git's history is editable. Basically, this allows you to change your workflow such that you can hack away at will, commit often and then rewrite the history of your hacking session so that you have a coherent set of patches/commits at the end of it with a useful changelog.

e.g. you can go from:

A1---B1---A2---A3---C1---B2---C2---C3

to:

A1---A2---A3---B1---B2---C1---C2---C3

or even:

A'---B'---C'

Using git rebasing, I found that I could use a similar workflow to using quilt with CVS, or mercurial with its patch queue (mq) extension. The revision history becomes less about tracking the progress of your work, and more a maleable mechanism for preparing patches before submitting upstream.

Red Hat Magazine has a nice article explaining all this, and I even picked up some new tricks to try out:

  • git-merge --squash : merge a branch/tag into the current branch, but squash all the commits together as an uncommitted change to the working tree. When you go to commit the result, the changelog of all the merged commits is available in the commit message editor so you can munge them together into a useful changelog.
  • git-cherry-pick --no-commit : apply the changes from a given commit to your working tree, but do not commit it. Could be used to achieve something similar to a squashed merge, but where you selectively merge only some of the commits.
  • git-add --patch/--interactive : add some changes from the working tree to the index, but e.g. selectively add only some of the patch hunks from a given file. Allows you to make a bunch of changes to a file, but commit the changes as individual commits.
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Fedora 9 Xen pv_ops

For the past couple of weeks, I've been helping out with the Fedora 9 pv_ops effort, specifically helping get the pv_ops based dom0 kernel going.

Well, following on from sct getting dom0 booting, I made a nice breakthrough this morning - a pv_ops dom0 booting a pv_ops domU:
$> dmesg | grep paravirt Booting paravirtualized kernel on Xen $> virsh create ./test-domu.xml Domain Test created from ./test-domu.xml $> virsh console Test | grep paravirt Booting paravirtualized kernel on Xen

What's this pv_ops business all about? Well, as Dan explained, for a long time we've been forward-porting Xensource's (now 2.6.18 based) kernel tree in an effort to try and have our Xen kernel not lag behind Fedora's bare-metal kernel. Now that the upstream kernel has gained the ability to run on Xen using pv_ops (but only as i386 DomU, currently) we've taken the decision to stop wasting our time forward porting Xensource's tree and put all our focus into improving the feature set of pv_ops based Xen.

pv_ops itself is a set of hooks in the kernel so that support for running on different hypervisors can be cleanly added to the kernel, with the added bonus that the kernel can detect at runtime which hypervisor it is running on and adapt itself accordingly. This means that, in the long run, Xen support should be more akin to a device driver than a huge fork of the kernel.

(Note: for any others who ever to debug Xen's booting of a guest, here's a tiny Xen domain builder)

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Dublin Marathon

Thanks to Olav, I can post here again after nearly 10 months (!). Not that I had anything to say anyway :-P

But for the past couple of months I've been writing about stuff like hiking, running and sailing on another blog and today's tidbit is that I finished my first marathon yesterday.


Happy Finisher

Woo!

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Virtual networking

Dan I have been discussing how to "fix virtual networking", not just Xen's networking but also getting something sane wrt. QEMU/KVM etc.

Anyone interested should read this writeup. To discuss, libvirt-list is probably the best place.

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QEMU Networking

QEMU has a number of really nice ways to set up networking for its guests. It can be a little bewildering to figure out how each of the options work, so I thought I'd write up what I found. Excuse the 'orrid ascii art :-)

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