On 08.04.2022 13:26, Andrew Cooper wrote: > 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.
Same on x86, afaict. >> 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.) That has changed not so long ago, Linux will now use all vCPU-s supplied by Xen. Since I was able to over-size Dom0 with XenoLinux, I wanted to have the ability also with the upstream version. Jan
