Messages in this thread | | | Date | Mon, 13 Dec 2021 15:26:58 +0100 | Subject | Re: [PATCH v5 1/1] s390x: KVM: accept STSI for CPU topology information | From | Pierre Morel <> |
| |
On 12/9/21 16:54, Heiko Carstens wrote: > On Thu, Dec 09, 2021 at 01:36:16PM +0100, Claudio Imbrenda wrote: >> On Mon, 22 Nov 2021 14:14:43 +0100 >> Pierre Morel <pmorel@linux.ibm.com> wrote: >> >>> We let the userland hypervisor know if the machine support the CPU >>> topology facility using a new KVM capability: KVM_CAP_S390_CPU_TOPOLOGY. >>> >>> 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 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. >>> >>> We assume in this patch: >>> - no polarization change: only horizontal polarization is currently >>> used in linux. > > Why is this assumption necessary? The statement that Linux runs only > with horizontal polarization is not true. >
Right, I will rephrase this as:
"Polarization change is not taken into account, QEMU intercepts queries for polarization change (PTF) and only provides horizontal polarization indication to Guest's Linux."
@Heiko, I did not find any usage of the polarization in the kernel other than an indication in the sysfs. Is there currently other use of the polarization that I did not see?
-- Pierre Morel IBM Lab Boeblingen
| |