Re: [PATCH 1/5] powerpc/pseries: do not use msgsndp doorbells on POWER9 guests

[Nicholas Piggin <npiggin@gmail.com>]

On Mon, 10 Apr 2017 14:07:38 +1000
Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:

> On Mon, 2017-04-10 at 13:22 +1000, Nicholas Piggin wrote:
> > On Sun, 09 Apr 2017 18:03:35 +1000  
> > > Benjamin Herrenschmidt <benh@kernel.crashing.org> wrote:  
> >   
> > > On Fri, 2017-04-07 at 22:55 +1000, Nicholas Piggin wrote:  
> > > > POWER9 hypervisors will not necessarily run guest threads together on
> > > > the same core at the same time, so msgsndp should not be used.    
> > > 
> > > Maybe we shouldn't advertise doorbells at all ?  
> > 
> > Possibly not. I don't know if there is a good way to switch it off
> > in the guest from the host with device tree currently. Might be better
> > to clear it early in the guest rather than test it here though?  
> 
> Sure for now. Long term a separate DT entry would be good.

cpufeatures will do that, but I wonder if some hypervisors won't implement
that binding then might be worth getting into an existing one in PAPR.

Anyway for now I'll make the change and have P9 guest clear the feature
themselves if you like.

> Might be
> worth re-enabling one day if we have HW virtualization of geust
> doorbells.

Yes. Also some hypervisor might run in a virtual sibling = physical
sibling mode on P9 too where it could still work.

Thanks,
Nick