Category Archives: Software

Learning ACPI for ARM64 part 1: Finding the Root.

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It started as a request from our tech lead: please help triage these patches. So I lookedat the set of patches and started with what looked like the simplest one:

Fix topology for Core scheduling.

It *just* reorders the code to call

store_cpu_topology(cpu);
before
notify_cpu_starting() .

Yeah…there is no such thing as a simple patch. These are my notes as I study and learn ACPI. My assumptions are here for all to see, and may well prove to be wrong.

Lets dig in.

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Using virt-install and cloud-init

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I want to call out a stellar article that told me exactly what I needed to do in order to use virt-install and cloud-init to launch a cloud-image. The only thing I have to add is the caveat that the #cloud-config comment at the top of the user-data file is required. The system will ignore the file if it does not start with that comment. This is the easiest way I know to launch a brand new VM.

XPath for libvirt external snapshop path

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The following xmllint XPath query will pull out the name of the backing file for a VM named fedora-server-36 and an external snapshot named fedora-36-post-install,

virsh snapshot-dumpxml fedora-server-36 fedora-server-36-post-install | xmllint --xpath "string(//domainsnapshot/disks/disk[@snapshot='external']/source/@file)" -

The string function extracts the attribute value.

This value can be used in the process of using or deleting the snapshot.

Copy in for-each loops in C++

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I had a bug in my OpenGL program. Here was the original code:

  for (Orbitor o : orbitors){
    o.calculate_position();
  }

and here was the working version

  for (std::vector<Orbitor>::iterator it = orbitors.begin() ;
       it != orbitors.end();
       ++it)
    {
      it->calculate_position();
    }
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Converting an OpenGL project to use Autotools

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In a science fiction game I play, we often find ourselves asking “which side of the Sun is Mars from the Earth right now?” Since this is for a game, the exact distance is not important, just “same side” or “90 degrees ahead” is a sufficient answer. But “right now” actually means a couple hundred years in the future.

Perfect problem to write a little code to solve. Here’s what it looks like.

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Parsing libvirt xmldump using xpath

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In a recent article, I saw yet another example of using grep to pull information out of xml, and then to manually look for a field. However, XML is structured, and with XPath, we can pull out exactly what we need.

virsh dumpxml fedora-server-36 | xmllint --xpath "//domain/devices/disk[@device='disk']"  -

That will produce output like this:

<disk type="file" device="disk">
      <driver name="qemu" type="qcow2" discard="unmap"/>
      <source file="/var/lib/libvirt/images/fedora-server-36.qcow2"/>
      <target dev="vda" bus="virtio"/>
      <address type="pci" domain="0x0000" bus="0x05" slot="0x00" function="0x0"/>
</disk>

Note that I did more in my XPath than required by the original article. I wanted to show an example of querying based on an attribute inside the selected node.

Update: Here is an example for what is done later in the article: pull the path out of the pool xml.

virsh pool-dumpxml default |  xmllint --xpath "//pool/target/path/text()"  -
/var/lib/libvirt/images