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@xxxxxxxxxxxxx> 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.
