Hardware discovery and kernel auto-configuration in MAAS

by Andres Rodriguez on 12 September 2019

In this blog, we are going to explore how to leverage MAAS for hardware discovery and kernel auto-configuration using tags.

In many cases, certain pieces of hardware require extra kernel parameters to be set in order to make use of them. For example, when configuring GPU passthrough we will typically need to configure the GPU card with specific kernel parameters. To achieve this, we will rely on MAAS’ hardware discovery, Xpath expressions and machine tags.

Tags, XPath expressions and kernel parameters

Machine tags is a mechanism used in MAAS to easily identify machines. While tags can be manually assigned to machines, they can also be automatically assigned if those machines match a specific pattern – the XPath expression – which describes the location of an element or an attribute in an XML document.

When commissioning a machine, MAAS gathers the lshw output (in XML) which lists all the information about the attached hardware. When creating a tag, MAAS allows to provide the XPath definition. This definition is then matched to the gathered lshw information. If this matches, the tag will be applied to all of the commissioned machines.

Similarly, when creating a tag one can specify which kernel parameters to apply to the machine by assigning the tag. Combining the definition and the kernel options in the single tag creation will allow MAAS to automatically discover all machines that match the XPath expression and automatically apply the kernel parameters once this machine is deployed. The following demonstrates the base command to use.

$ maas <username> tags create \
    definition=’<XPath expression>’ \
    kernel_opts=’<Kernel parameters>’

A practical example

As a practical example, we want to configure GPU passthrough. For this, we want to create a tag that automatically matches all machines with Intel VT-d enabled and have a Tesla v100 PCIe 16GB GPU. We do so by using a definition similar to:

definition='//node[@id="cpu:0"]/capabilities/capability/@id = "vmx" and //node[@id="display"]/vendor[contains(.,"NVIDIA")] and //node[@id="display"]/description[contains(.,"3D")] and //node[@id="display"]/product[contains(.,"Tesla V100 PCIe 16GB")]'

But since we want this to be configured at deployment time, we want to set the kernel parameters to apply on a deployed machine:

kernel_opts="nomodeset modprobe.blacklist=nouveau,nvidiafb,snd_hda_intel nouveau.blacklist=1 nouveau.blacklist=1 nouveau.blacklist=1 video=vesafb:off,efifb:off intel_iommu=on rd.driver.pre=pci-stub rd.driver.pre=vfio-pci pci-stub.ids=10de:1db4 vfio-pci.ids=10de:1db4 vfio_iommu_type1.allow_unsafe_interrupts=1 vfio-pci.disable_vga=1"

These kernel parameters will:

• Blacklist drivers and disable displays
• Enable IOMMU 
• Pre-load kernel modules
• And reserve PCI ID (10de:1db4) for GPU Passthrough

As such, creating a tag that will auto-apply to all machines that match the hardware definition and apply kernel parameters at deployment time will look like this:

$ maas <username> tags create name=gpgpu-tesla-vi \
     comment="Enable passthrough for Nvidia Tesla V series GPUs 
              on Intel" \
     definition='
         //node[@id="cpu:0"]/capabilities/capability/@id = "vmx" 
        and //node[@id="display"]/vendor[contains(.,"NVIDIA")] 
        and //node[@id="display"]/description[contains(.,"3D")] 
        and //node[@id="display"]/product[contains(.,"Tesla V100 
        PCIe 16GB")]' \
     kernel_opts="console=tty0 console=ttyS0,115200n8r nomodeset 
          modprobe.blacklist=nouveau,nvidiafb,snd_hda_intel 
          nouveau.blacklist=1 nouveau.blacklist=1 
          nouveau.blacklist=1 video=vesafb:off,efifb:off 
          intel_iommu=on rd.driver.pre=pci-stub 
          rd.driver.pre=vfio-pci pci-stub.ids=10de:1db4
          vfio-pci.ids=10de:1db4 
          vfio_iommu_type1.allow_unsafe_interrupts=1
          vfio-pci.disable_vga=1"

Once this tag is created, every time a new machine is commissioned MAAS will automatically apply this tag if machines match the definition, allowing administrators to configure their homogeneous hardware at scale by simply defining a few set of tags.

For more information, please contact us or visit https://maas.io/docs/tags .