Subject: Re: [RFC][PATCH 0/2] reworking cause_ipi and adding global doorbell support

From: Benjamin Herrenschmidt <[email-masked]>
Date: Tue, 14 Mar 2017 10:31:08 +1100

On Mon, 2017-03-13 at 03:13 +1000, Nicholas Piggin wrote:
> Hi,
> 
> Just after the previous two fixes, I would like to propose changing
> the way we do doorbell vs interrupt controller IPIs, and add support
> for global doorbells supported by POWER9 in HV mode.
> 
> After this, the platform code knows about doorbells and interrupt
> controller IPIs, rather than they know about each other.

A few things come to mind:

 - We don't want to use doorbells under KVM. They are going to turn
into traps and be emulated, slower than using H_IPI, at least on P9.
Even for core only doorbells. I'm not sure how to convey that to the
guest.

 - On PP9 DD1 we need a CI load instead of msgsync (a DARN instruction
would do too if it works)

 - Can we get rid of the atomic ops for manipulating the IPI mux ? What
about a cache line per message and just set/clear ? If we clear in the
doorbell handler before we call the respective targets, we shouldn't
"lose" messages no ? As long as the actual handlers "loop" as necessary
of course.

Cheers,
Ben.

> Thanks,
> Nick
> 
> Nicholas Piggin (2):
>   powerpc/64s: change the doorbell IPI calling convention
>   powerpc/64s: use global doorbell on POWER9 in HV mode
> 
>  arch/powerpc/include/asm/dbell.h       | 38 ++++++++++++++++++++++
> -----
>  arch/powerpc/include/asm/smp.h         |  4 +--
>  arch/powerpc/include/asm/xics.h        |  2 +-
>  arch/powerpc/kernel/dbell.c            | 47 ++++++++++++++++++++++
> ------------
>  arch/powerpc/kernel/smp.c              | 27 ++++++++++---------
>  arch/powerpc/platforms/85xx/smp.c      |  9 +------
>  arch/powerpc/platforms/powermac/smp.c  |  2 +-
>  arch/powerpc/platforms/powernv/smp.c   | 32 +++++++++++++++++------
>  arch/powerpc/platforms/pseries/smp.c   | 28 ++++++++------------
>  arch/powerpc/sysdev/xics/icp-hv.c      |  2 +-
>  arch/powerpc/sysdev/xics/icp-native.c  | 12 +--------
>  arch/powerpc/sysdev/xics/icp-opal.c    |  2 +-
>  arch/powerpc/sysdev/xics/xics-common.c |  3 ---
>  13 files changed, 118 insertions(+), 90 deletions(-)
>