Re: [PATCH 6/6] x86/kvm: add boot parameter for setting max number of vcpus per guest

[Juergen Gross]

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


Juergen

Attachment: OpenPGP_0xB0DE9DD628BF132F.asc
Description: OpenPGP public key

Attachment: OpenPGP_signature
Description: OpenPGP digital signature