Juergen Gross <jgross@xxxxxxxx> writes: > On 14.07.21 15:21, Vitaly Kuznetsov wrote: >> Juergen Gross <jgross@xxxxxxxx> writes: >> >>> On 14.07.21 13:45, Vitaly Kuznetsov wrote: >>> >>>> Personally, I'd vote for introducing a 'ratio' parameter then so >>>> generally users will only have to set 'kvm.max_vcpus'. >>> >>> Okay. >>> >>> Default '4' then? Or '2 ^ (topology_levels - 2)' (assuming a >>> topology_level of 3 on Intel: thread/core/socket and 4 on EPYC: >>> thread/core/package/socket). >> >> I'd suggest we default to '4' for both Intel and AMD as we haven't given >> up completely on cross-vendor VMs (running AMD VMs on Intel CPUs and >> vice versa). It would be great to leave a comment where the number comes >> from of course. >> > > Thinking more about it I believe it would be better to make the > parameter something like "additional vcpu-id bits" with a default of > topology_levels - 2 (cross-vendor VMs are so special that I think the > need to specify another value explicitly in this case is acceptable). > > Reasons are: > > - the ability to specify factor values not being a power of 2 is weird > - just specifying the additional number of bits would lead to compatible > behavior (e.g. a max vcpu-id of 1023 with max_vcpus being 288 and the > default value of 1) > - the max vcpu-id should (normally) be 2^n - 1 Sounds good to me! Also, there's an ongoing work to raise the default KVM_MAX_VCPUS number by Eduardo (Cc): https://lore.kernel.org/kvm/20210831204535.1594297-1-ehabkost@xxxxxxxxxx/ It would be great if you could unify your efforts) -- Vitaly
