When the VCPU target of an H_IPI hypercall is not running
in the guest, we need to do a kick VCPU (wake the VCPU thread)
to make it runnable. The real-mode version of the H_IPI hypercall
cannot do this because it involves waking a sleeping thread.
Thus the hcall returns H_TOO_HARD which forces a switch back
to host so that the H_IPI call can be completed in virtual mode.
This has been found to cause a slowdown for many workloads like
YCSB MongoDB, small message networking, etc. 

This patch set optimizes the wakeup of the target VCPU by posting
a free core already running in the host to do the wakeup, thus
avoiding the switch to host and back. It requires maintaining a
bitmask of all the available cores in the system to indicate if
they are in the host or running in some guest. It also requires
the H_IPI hypercall to search for a free host core and send it a
new IPI message PPC_MSG_RM_HOST_ACTION after stashing away some
parameters like the pointer to VCPU for the IPI handler. Locks
are avoided by using atomic operations to save core state, to
find and reserve a core in the host, etc.

Note that it is possible for a guest to be destroyed and its
VCPUs freed before the IPI handler gets to run. This case is
handled by ensuring that any pending PPC_MSG_RM_HOST_ACTION
IPIs are completed before proceeding with freeing the VCPUs.

A tunable h_ipi_redirect is also included in the patch set to
disable the feature. 

This patch set depends upon patches to powerpc to increase the
number of supported IPI messages to 8 and which also defines

Suresh Warrier (6):
  KVM: PPC: Book3S HV: Host-side RM data structures
  KVM: PPC: Book3S HV: Manage core host state
  KVM: PPC: Book3S HV: kvmppc_host_rm_ops - handle offlining CPUs
  KVM: PPC: Book3S HV: Host side kick VCPU when poked by real-mode KVM
  KVM: PPC: Book3S HV: Send IPI to host core to wake VCPU
  KVM: PPC: Book3S HV: Add tunable to control H_IPI redirection

 arch/powerpc/include/asm/kvm_ppc.h   |  33 +++++++
 arch/powerpc/kernel/smp.c            |  11 +++
 arch/powerpc/kvm/book3s_hv.c         | 166 +++++++++++++++++++++++++++++++++++
 arch/powerpc/kvm/book3s_hv_builtin.c |   3 +
 arch/powerpc/kvm/book3s_hv_rm_xics.c | 120 ++++++++++++++++++++++++-
 arch/powerpc/kvm/powerpc.c           |  10 +++
 6 files changed, 340 insertions(+), 3 deletions(-)


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