On 14.07.21 15:21, Vitaly Kuznetsov wrote:
> Juergen Gross <jgross@suse.com> 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


Juergen