Re: l3 cache and cpu pinning

On Wed, Apr 21, 2021 at 12:53:49PM +0200, Roman Mohr wrote: > Hi, > > I have a question regarding enabling l3 cache emulation on Domains. Can > this also be enabled without cpu-pinning, or does it need cpu pinning to > emulate the l3 caches according to the cpus where the guest is pinned to? I presume you're referring to <cpu> <cache level='3' mode='emulate|passthrough|none'/> </cpu> There is no hard restriction placed on usage of these modes by QEMU. Conceptually though, you only want to use "passthrough" mode if you have configured the sockets/cores/threads topology to match the host CPUs. In turn you only ever want to set sockets/cores/threads to match the host if you have done CPU pinning such that the topology actually matches the host CPUs that have been pinned to. As a rule of thumb - If letting CPUs float -> Always uses sockets=1, cores=num-vCPUs, threads=1 -> cache==emulate -> Always use 1 guest NUMA node (ie the default) - If strictly pinning CPUs 1:1 -> Use sockets=N, cores=M, threads=0 to match the topology of the CPUs that have been pinned to

-> cache==passthrough -> Configure virtual NUMA nodes if the CPU pinning or guest RAM needs cross host NUMA nodes.