On 08/04/2022 12:08, Julien Grall wrote: > Hi, > > On 08/04/2022 12:01, Roger Pau Monné wrote: >>>> I could add a suitable dom0_max_vcpus parameter to osstest. XenServer >>>> uses 16 for example. >>> >>> I'm afraid a fixed number won't do, the more that iirc there are >>> systems with just a few cores in the pool (and you don't want to >>> over-commit by default). >> >> But this won't over commit, it would just assign dom0 16 vCPUs at >> most, if the system has less than 16 vCPUs that's what would be >> assigned to dom0. > > AFAICT, this is not the case on Arm. If you ask 16 vCPUs, then you > will get that number even if there are 8 pCPUs. > > In fact, the documentation of dom0_max_vcpus suggests that the numbers > of vCPUs can be more than the number of pCPUs.
XenServer uses dom0_max_vcpus=1-16 so we dont oversubscribe (even if CPUs get turned off in firmware), but top out at 16. It is possible to use this option to create more vcpus, but whether dom0 decides to do anything with them is up to dom0. Linux won't go any further than it can see CPUs listed in the ACPI tables (and yes, this is a host/guest laying violation for PV dom0 where dom0 sees the system ACPI tables.) ~Andrew
