Re: [PATCH v2 10/14] KVM: PPC: Book3S HV: POWER9 does not require secondary thread management

[Paul Mackerras <paulus@ozlabs.org>]

On Sat, Aug 12, 2017 at 02:39:08AM +1000, Nicholas Piggin wrote:
> POWER9 CPUs have independent MMU contexts per thread, so KVM does not
> need to quiesce secondary threads, so the hwthread_req/hwthread_state
> protocol does not have to be used. So patch it away on POWER9, and patch
> away the branch from the Linux idle wakeup to kvm_start_guest that is
> never used.

If/when we add support for running HPT guests on a radix host, we will
have to run the host in single-threaded mode (since POWER9 doesn't
support having some threads of a core using HPT and some using radix
simultaneously).  We'll then need some sort of thing like
kvmppc_grab_hwthread to coordinate with the threads so that guests can
use the secondary threads.

So I think most of this code should stay.  We will still need to have
a way to make sure that the secondaries are in real mode and not in a
guest, because all threads will need to be in real mode when switching
the core between radix and HPT mode.  Maybe we can optimize it a bit
at present given that we don't yet support running HPT guests on a
radix host, but I don't want to make it harder to do that in future.

Paul.