[PATCH v5 0/1] s390x: KVM: CPU Topology

From: Pierre Morel
Date: Mon Nov 22 2021 - 08:14:15 EST

Hi all,

This new series add the implementation of interpretation for
the PTF instruction.

The series provides:
1- interception of the STSI instruction forwarding the CPU topology
2- interpretation of the PTF instruction
3- a KVM capability for the userland hypervisor to ask KVM to
setup PTF interpretation.

0- Foreword

The S390 CPU topology is reported using two instructions:
- PTF, to get information if the CPU topology did change since last
PTF instruction or a subsystem reset.
- STSI, to get the topology information, consisting of the topology
of the CPU inside the sockets, of the sockets inside the books etc.

The PTF(2) instruction report a change if the STSI(15.1.2) instruction
will report a difference with the last STSI(15.1.2) instruction*.
With the SIE interpretation, the PTF(2) instruction will report a
change to the guest if the host sets the SCA.MTCR bit.

*The STSI(15.1.2) instruction reports:
- The cores address within a socket
- The polarization of the cores
- The CPU type of the cores
- If the cores are dedicated or not

We decided to implement the CPU topology for S390 in several steps:
- first step we provide a correct topology only for dedicated CPUs
and vCPUs. We provide the basic framework and report topology change
only when a new vCPU is plugged in a monotonic scheme.

In future development we will provide:
- NUMA handling, allowing holes inside the cores bitmap reported by
- dedicated versus shared CPUs handling
- vCPU migration on a different CPU

We will ignore the following changes inside a STSI(15.1.2):
- polarization: only horizontal polarization is currently used in Linux.
- CPU Type: only IFL Type are supported in Linux

1- Interception of STSI

To provide Topology information to the guest through the STSI
instruction, we forward STSI with Function Code 15 to the
userland hypervisor which will take care to provide the right
information to the guest.

To let the guest use both the PTF instruction to check if a topology
change occurred and sthe STSI_15.x.x instruction we add a new KVM
capability to enable the topology facility.

2- Interpretation of PTF with FC(2)

The PTF instruction will report a topology change if there is any change
with a previous STSI(15.1.2) SYSIB.
Changes inside a STSI(15.1.2) SYSIB occur if CPU bits are set or clear
inside the CPU Topology List Entry CPU mask field, which happens with
changes in CPU polarization, dedication, CPU types and adding or
removing CPUs in a socket.

The reporting to the guest is done using the Multiprocessor
Topology-Change-Report (MTCR) bit of the utility entry of the guest's
SCA which will be cleared during the interpretation of PTF.

To check if the topology has been modified we use a new field of the
arch vCPU prev_cpu, to save the previous real CPU ID at the end of a
schedule and verify on next schedule that the CPU used is in the same
socket, this field is initialized to -1 on vCPU creation.


Pierre Morel (1):
s390x: KVM: accept STSI for CPU topology information

Documentation/virt/kvm/api.rst | 16 ++++++++++
arch/s390/include/asm/kvm_host.h | 14 ++++++---
arch/s390/kvm/kvm-s390.c | 52 +++++++++++++++++++++++++++++++-
arch/s390/kvm/priv.c | 7 ++++-
arch/s390/kvm/vsie.c | 3 ++
include/uapi/linux/kvm.h | 1 +
6 files changed, 87 insertions(+), 6 deletions(-)



from v4 tp v5

- modify the way KVM_CAP is tested to be OK with vsie

from v3 to v4

- squatch both patches

- Added Documentation

- Modified the detection for new vCPUs

from v2 to v3

- use PTF interpretation

- optimize arch_update_cpu_topology using PTF

from v1 to v2:

- Add a KVM capability to let QEMU know we support PTF and STSI 15

- check KVM facility 11 before accepting STSI fc 15

- handle all we can in userland

- add tracing to STSI fc 15