Messages in this thread | | | Subject | Re: [PATCH v3 2/3] s390x: KVM: Implementation of Multiprocessor Topology-Change-Report | From | Pierre Morel <> | Date | Wed, 8 Sep 2021 14:52:04 +0200 |
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On 9/8/21 2:01 PM, Christian Borntraeger wrote: > > > On 08.09.21 14:00, Pierre Morel wrote: >> >> >> On 9/8/21 9:04 AM, Christian Borntraeger wrote: >>> >>> >>> On 07.09.21 12:24, Pierre Morel wrote: >>>> >>>> >>>> On 9/6/21 8:37 PM, David Hildenbrand wrote: >>>>> On 03.08.21 10:26, Pierre Morel 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_2 SYSIB. >>>>>> Changes inside a STSI_15_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 deliberatly ignore: >>>>>> - polarization: only horizontal polarization is currently used in >>>>>> linux. >>>>>> - CPU Type: only IFL Type are supported in Linux >>>>>> - Dedication: we consider that only a complete dedicated CPU stack >>>>>> can >>>>>> take benefit of the CPU Topology. >>>>>> >>>>>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com> >>>>> >>>>> >>>>>> @@ -228,7 +232,7 @@ struct kvm_s390_sie_block { >>>>>> __u8 icptcode; /* 0x0050 */ >>>>>> __u8 icptstatus; /* 0x0051 */ >>>>>> __u16 ihcpu; /* 0x0052 */ >>>>>> - __u8 reserved54; /* 0x0054 */ >>>>>> + __u8 mtcr; /* 0x0054 */ >>>>>> #define IICTL_CODE_NONE 0x00 >>>>>> #define IICTL_CODE_MCHK 0x01 >>>>>> #define IICTL_CODE_EXT 0x02 >>>>>> @@ -246,6 +250,7 @@ struct kvm_s390_sie_block { >>>>>> #define ECB_TE 0x10 >>>>>> #define ECB_SRSI 0x04 >>>>>> #define ECB_HOSTPROTINT 0x02 >>>>>> +#define ECB_PTF 0x01 >>>>> >>>>> From below I understand, that ECB_PTF can be used with stfl(11) in >>>>> the hypervisor. >>>>> >>>>> What is to happen if the hypervisor doesn't support stfl(11) and we >>>>> consequently cannot use ECB_PTF? Will QEMU be able to emulate PTF >>>>> fully? >>>> >>>> Yes. >>> >>> Do we want that? I do not think so. Other OSes (like zOS) do use PTF >>> in there low level interrupt handler, so PTF must be really fast. >>> I think I would prefer that in that case the guest will simply not >>> see stfle(11). >>> So the user can still specify the topology but the guest will have no >>> interface to query it. >> >> I do not understand. >> If the host support stfle(11) we interpret PTF. >> >> The proposition was to emulate only in the case it is not supported, >> what you propose is to not advertise stfl(11) if the host does not >> support it, and consequently to never emulate is it right? > > Yes, exactly. My idea is to provide it to guests if we can do it fast, > but do not provide it if it would add a performance issue.
OK, understood, I will update this and the QEMU part too as we do not need emulation there anymore.
Thanks, Pierre
-- Pierre Morel IBM Lab Boeblingen
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