Hi all: As we are know Intel® Xeon phi targets high-performance computing and other parallel workloads. Now qemu has supported phi virtualization，it is time for libvirt to support phi. Different from the traditional X86 server, There is a special numa node with Multi-Channel DRAM (MCDRAM) on Phi, but without any CPU . Now libvirt requires nonempty cpus argument for NUMA node, such as. <numa> <cell id='0' cpus='0-239' memory='80' unit='GiB'/> <cell id='1' cpus='240-243' memory='16' unit='GiB'/> </numa> In order to support phi virtualization, libvirt needs to allow a numa cell definition without 'cpu' attribution. Such as: <numa> <cell id='0' cpus='0-239' memory='80' unit='GiB'/> <cell id='1' memory='16' unit='GiB'/> </numa> When a cell without 'cpu', qemu will allocate memory by default MCDRAM instead of DDR. Now here I'd like to discuss these questions: 1. This feature is only for Phi at present, but we will check Phi platform for CPU-less NUMA node. The NUMA node without CPU indicates MCDRAM node. And for now MCDRAM is available only for PHI. However, there is no reason that any other platform couldn’t define CPU-less NUMA node using libvirt, so there is no reason to check if PHI is used or not. 2. Type of memory of CPU-less NUMA node will not be checked during machine creation/configuration step. There is no reliable way to distinguish if the node is MCDRAM or regular DDR. This step is not concerned with type of the memory, only with NUMA assignment. 3. Unlike traditional memory assign to a VM, MCDRAM do not support over commit
If the memory of a virtual NUMA node is going to be explicitly bound to physical NUMA node then it shouldn’t exceed the size of its corresponding NUMA node, doesn’t matter if it is MCDRAM or DDR.
4. Make sure at least one numa must include a CPU info field be passed to qemu.
At least one NUMA node must define list of CPUs assigned, or if none has it assigned by default assign all CPUs to virtual NUMA node 0.