KVM: PPC: Book3S HV: XIVE: Add a comment regarding VP numbering

KVM: PPC: Book3S HV: XIVE: Add a comment regarding VP numbering

When the XIVE resources are allocated at the HW level, the VP
structures describing the vCPUs of a guest are distributed among
the chips to optimize the PowerBUS usage. For best performance, the
guest vCPUs can be pinned to match the VP structure distribution.

Currently, the VP identifiers are deduced from the vCPU id using
the kvmppc_pack_vcpu_id() routine which is not incorrect but not
optimal either. It VSMT is used, the result is not continuous and
the constraints on HW resources described above can not be met.

Signed-off-by: Cédric Le Goater <clg@kaod.org>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20201210171450.1933725-14-clg@kaod.org