About Adam Young

Once upon a time I was an Army Officer, but that was long ago. Now I work as a Software Engineer. I climb rocks, play saxophone, and spend way too much time in front of a computer.

XPath for libvirt external snapshop path

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++

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

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

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

Building Linux tip-of-tree on an Ampere based system

I Have an Ampere Altra-Max/INGRASYS Yushan Server System running Centos 8 stream.

Because we are a chip manufacteror, we don’t sell end systems, we provide a reference platform that is a starting point for our customers to make a product. This leads to bizarre set of internal versus external names. One thing that you can rely on, however, is the identifier of the processor itself:

# cat /proc/cpuinfo 
processor	: 0
BogoMIPS	: 50.00
Features	: fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm lrcpc dcpop asimddp ssbs
CPU implementer	: 0x41
CPU architecture: 8
CPU variant	: 0x3
CPU part	: 0xd0c
CPU revision	: 1
...

TO make this readable, use the utility lscpu:

[root@eng14sys-r
111 ~]# lscpu 
Architecture:        aarch64
Byte Order:          Little Endian
CPU(s):              80
On-line CPU(s) list: 0-79
Thread(s) per core:  1
Core(s) per socket:  80
Socket(s):           1
NUMA node(s):        1
Vendor ID:           ARM
BIOS Vendor ID:      Ampere(R)
Model:               1
Model name:          Neoverse-N1
BIOS Model name:     Ampere(R) Altra(R) Processor
Stepping:            r3p1
CPU max MHz:         3000.0000
CPU min MHz:         1000.0000
BogoMIPS:            50.00
L1d cache:           64K
L1i cache:           64K
L2 cache:            1024K
NUMA node0 CPU(s):   0-79
Flags:               fp asimd evtstrm aes pmull sha1 sha2 crc32 atomics fphp asimdhp cpuid asimdrdm lrcpc dcpop asimddp ssbs


I want to build the latest Linus-repo Linux Kernel and run it on the server. Here’s the steps I went through.

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ipxe.efi for aarch64

To make the AARCH64 ipxe process work using bifrost, I had to

git clone https://github.com/ipxe/ipxe.git
cd ipxe/src/
make bin-arm64-efi/snponly.efi ARCH=arm64
sudo cp bin-arm64-efi/snponly.efi /var/lib/tftpboot/ipxe.efi

This works for the Ampere reference implementation servers that use a Mellanox network interface card, which supports (only) snp.