[v8,0/2] s390x: KVM: CPU Topology

Message

Pierre Morel April 20, 2022, 11:34 a.m. UTC

Hi all,

This new spin adds reset handling and bug correction to the series
for 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 we report CPU hotplug
- modification of the CPU mask inside sockets

In future development we will provide:

- handling of shared CPUs
- reporting of the CPU Type
- reporting of the polarization


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.


Regards,
Pierre


Pierre Morel (2):
  s390x: KVM: guest support for topology function
  s390x: KVM: resetting the Topology-Change-Report

 Documentation/virt/kvm/api.rst   |  16 +++
 arch/s390/include/asm/kvm_host.h |  12 ++-
 arch/s390/include/uapi/asm/kvm.h |   9 ++
 arch/s390/kvm/kvm-s390.c         | 177 ++++++++++++++++++++++++++++++-
 arch/s390/kvm/kvm-s390.h         |  25 +++++
 arch/s390/kvm/priv.c             |  14 ++-
 arch/s390/kvm/vsie.c             |   3 +
 include/uapi/linux/kvm.h         |   1 +
 8 files changed, 249 insertions(+), 8 deletions(-)