On Thu, 2017-08-10 at 14:28 +1000, David Gibson wrote: > > Also, will POWER9 always have doorbells? In which case you could > reduce it to 3 options.
The problem with doorbells on POWER9 guests is that they may have to trap and be emulated by the hypervisor, since the guest threads on P9 don't have to match the HW threads of the core. Thus it's quite possible that using XIVE for IPIs is actually faster than doorbells in that case. Cheers, Ben.