Re: [PATCH v2 09/10] capabilities: Expose NUMA interconnects

Links between NUMA nodes can have different latencies and bandwidths. This info is newly defined in ACPI 6.2 under Heterogeneous Memory Attribute Table (HMAT) table. Linux kernel learned how to report these values under sysfs and thus we can expose them in our capabilities XML. The sysfs interface is documented in kernel's Documentation/admin-guide/mm/numaperf.rst. Long story short, two nodes can be in initiator-target relationship. A node can be initiator if it has a CPU or a device that's capable of initiating memory transfer. Therefore a node that has just memory can only be target. An initiator-target link can then have any combination of {bandwidth, latency} - {access, read, write} attribute (6 in total). However, the standard says access is applicable iff read and write values are the same. Therefore, we really have just four combinations of attributes: bandwidth-read, bandwidth-write, latency-read, latency-write. This is the combination that kernel reports anyway. Then, under /sys/system/devices/node/nodeX/acccessN/initiators we find values for those 4 attributes and also symlinks named "nodeN" which then represent initiators to nodeX. For instance: /sys/system/node/node1/access1/initiators/node0 -> ../../node0 /sys/system/node/node1/access1/initiators/read_bandwidth /sys/system/node/node1/access1/initiators/read_latency /sys/system/node/node1/access1/initiators/write_bandwidth /sys/system/node/node1/access1/initiators/write_latency This means that node0 is initiator and node1 is target and values of the interconnect can be read. In theory, there can be separate links to memory side caches too (e.g. one link from node X to node Y's main memory, another from node X to node Y's L1 cache, another one to L2 cache and so on). But sysfs does not express this relationship just yet. The "accessN" means either "access0" or "access1". The difference is that while the former expresses the best interconnect between two nodes including CPUS and I/O devices (such as GPUs and NICs), the latter includes only CPUs and thus is what we need. Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1786309 Signed-off-by: Michal Privoznik <mprivozn(a)redhat.com&gt;