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Subject: Re: [PATCH v3 2/3] s390x: KVM: Implementation of Multiprocessor
 Topology-Change-Report
To:     Christian Borntraeger <borntraeger@de.ibm.com>,
        David Hildenbrand <david@redhat.com>, kvm@vger.kernel.org
Cc:     linux-s390@vger.kernel.org, linux-kernel@vger.kernel.org,
        frankja@linux.ibm.com, cohuck@redhat.com, thuth@redhat.com,
        imbrenda@linux.ibm.com, hca@linux.ibm.com, gor@linux.ibm.com
References: <1627979206-32663-1-git-send-email-pmorel@linux.ibm.com>
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 <c4cfc6b1-44b5-eda7-c602-a54858971f01@linux.ibm.com>
 <720762c5-01ab-55f6-e585-b46c9b62ce8c@de.ibm.com>
 <9e062d47-ef96-4028-38a7-ffb54a451643@linux.ibm.com>
 <2952e19f-2bb4-35d1-b3dd-042fbb08f9eb@de.ibm.com>
From:   Pierre Morel <pmorel@linux.ibm.com>
Message-ID: <98a3de32-0ac5-00f3-8799-dfff8edae70d@linux.ibm.com>
Date:   Wed, 8 Sep 2021 16:17:22 +0200
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On 9/8/21 3:16 PM, Christian Borntraeger wrote:
> 
> 
> On 08.09.21 15:09, Pierre Morel wrote:
>>
>>
>> On 9/8/21 9:07 AM, Christian Borntraeger wrote:
>>>
>>>
>>> On 07.09.21 14:28, 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?
>>>>>
>>>>>
>>>>>>       __u8    ecb;            /* 0x0061 */
>>>>>>   #define ECB2_CMMA    0x80
>>>>>>   #define ECB2_IEP    0x20
>>>>>> @@ -747,6 +752,7 @@ struct kvm_vcpu_arch {
>>>>>>       bool skey_enabled;
>>>>>>       struct kvm_s390_pv_vcpu pv;
>>>>>>       union diag318_info diag318_info;
>>>>>> +    int prev_cpu;
>>>>>>   };
>>>>>>   struct kvm_vm_stat {
>>>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>>>> index b655a7d82bf0..ff6d8a2b511c 100644
>>>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>>>> @@ -568,6 +568,7 @@ int kvm_vm_ioctl_check_extension(struct kvm 
>>>>>> *kvm, long ext)
>>>>>>       case KVM_CAP_S390_VCPU_RESETS:
>>>>>>       case KVM_CAP_SET_GUEST_DEBUG:
>>>>>>       case KVM_CAP_S390_DIAG318:
>>>>>> +    case KVM_CAP_S390_CPU_TOPOLOGY:
>>>>>
>>>>> I would have expected instead
>>>>>
>>>>> r = test_facility(11);
>>>>> break
>>>>>
>>>>> ...
>>>>>
>>>>>>           r = 1;
>>>>>>           break;
>>>>>>       case KVM_CAP_SET_GUEST_DEBUG2:
>>>>>> @@ -819,6 +820,23 @@ int kvm_vm_ioctl_enable_cap(struct kvm *kvm, 
>>>>>> struct kvm_enable_cap *cap)
>>>>>>           icpt_operexc_on_all_vcpus(kvm);
>>>>>>           r = 0;
>>>>>>           break;
>>>>>> +    case KVM_CAP_S390_CPU_TOPOLOGY:
>>>>>> +        mutex_lock(&kvm->lock);
>>>>>> +        if (kvm->created_vcpus) {
>>>>>> +            r = -EBUSY;
>>>>>> +        } else {
>>>>>
>>>>> ...
>>>>> } else if (test_facility(11)) {
>>>>>      set_kvm_facility(kvm->arch.model.fac_mask, 11);
>>>>>      set_kvm_facility(kvm->arch.model.fac_list, 11);
>>>>>      r = 0;
>>>>> } else {
>>>>>      r = -EINVAL;
>>>>> }
>>>>>
>>>>> similar to how we handle KVM_CAP_S390_VECTOR_REGISTERS.
>>>>>
>>>>> But I assume you want to be able to support hosts without ECB_PTF, 
>>>>> correct?
>>>>>
>>>>>
>>>>>> +            set_kvm_facility(kvm->arch.model.fac_mask, 11);
>>>>>> +            set_kvm_facility(kvm->arch.model.fac_list, 11);
>>>>>> +            r = 0;
>>>>>> +        }
>>>>>> +        mutex_unlock(&kvm->lock);
>>>>>> +        VM_EVENT(kvm, 3, "ENABLE: CPU TOPOLOGY %s",
>>>>>> +             r ? "(not available)" : "(success)");
>>>>>> +        break;
>>>>>> +
>>>>>> +        r = -EINVAL;
>>>>>> +        break;
>>>>>
>>>>> ^ dead code
>>>>>
>>>>> [...]
>>>>>
>>>>>>   }
>>>>>>   void kvm_arch_vcpu_put(struct kvm_vcpu *vcpu)
>>>>>>   {
>>>>>> +    vcpu->arch.prev_cpu = vcpu->cpu;
>>>>>>       vcpu->cpu = -1;
>>>>>>       if (vcpu->arch.cputm_enabled && !is_vcpu_idle(vcpu))
>>>>>>           __stop_cpu_timer_accounting(vcpu);
>>>>>> @@ -3198,6 +3239,11 @@ static int kvm_s390_vcpu_setup(struct 
>>>>>> kvm_vcpu *vcpu)
>>>>>>           vcpu->arch.sie_block->ecb |= ECB_HOSTPROTINT;
>>>>>>       if (test_kvm_facility(vcpu->kvm, 9))
>>>>>>           vcpu->arch.sie_block->ecb |= ECB_SRSI;
>>>>>> +
>>>>>> +    /* PTF needs both host and guest facilities to enable 
>>>>>> interpretation */
>>>>>> +    if (test_kvm_facility(vcpu->kvm, 11) && test_facility(11))
>>>>>> +        vcpu->arch.sie_block->ecb |= ECB_PTF;
>>>>>
>>>>> Here you say we need both ...
>>>>>
>>>>>> +
>>>>>>       if (test_kvm_facility(vcpu->kvm, 73))
>>>>>>           vcpu->arch.sie_block->ecb |= ECB_TE;
>>>>>> diff --git a/arch/s390/kvm/vsie.c b/arch/s390/kvm/vsie.c
>>>>>> index 4002a24bc43a..50d67190bf65 100644
>>>>>> --- a/arch/s390/kvm/vsie.c
>>>>>> +++ b/arch/s390/kvm/vsie.c
>>>>>> @@ -503,6 +503,9 @@ static int shadow_scb(struct kvm_vcpu *vcpu, 
>>>>>> struct vsie_page *vsie_page)
>>>>>>       /* Host-protection-interruption introduced with ESOP */
>>>>>>       if (test_kvm_cpu_feat(vcpu->kvm, KVM_S390_VM_CPU_FEAT_ESOP))
>>>>>>           scb_s->ecb |= scb_o->ecb & ECB_HOSTPROTINT;
>>>>>> +    /* CPU Topology */
>>>>>> +    if (test_kvm_facility(vcpu->kvm, 11))
>>>>>> +        scb_s->ecb |= scb_o->ecb & ECB_PTF;
>>>>>
>>>>> but here you don't check?
>>>>>
>>>>>>       /* transactional execution */
>>>>>>       if (test_kvm_facility(vcpu->kvm, 73) && wants_tx) {
>>>>>>           /* remap the prefix is tx is toggled on */
>>>>>> diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
>>>>>> index d9e4aabcb31a..081ce0cd44b9 100644
>>>>>> --- a/include/uapi/linux/kvm.h
>>>>>> +++ b/include/uapi/linux/kvm.h
>>>>>> @@ -1112,6 +1112,7 @@ struct kvm_ppc_resize_hpt {
>>>>>>   #define KVM_CAP_BINARY_STATS_FD 203
>>>>>>   #define KVM_CAP_EXIT_ON_EMULATION_FAILURE 204
>>>>>>   #define KVM_CAP_ARM_MTE 205
>>>>>> +#define KVM_CAP_S390_CPU_TOPOLOGY 206
>>>>>
>>>>> We'll need a Documentation/virt/kvm/api.rst description.
>>>>>
>>>>> I'm not completely confident that the way we're handling the 
>>>>> capability+facility is the right approach. It all feels a bit 
>>>>> suboptimal.
>>>>>
>>>>> Except stfl(74) -- STHYI --, we never enable a facility via 
>>>>> set_kvm_facility() that's not available in the host. And STHYI is 
>>>>> special such that it is never implemented in hardware.
>>>>>
>>>>> I'll think about what might be cleaner once I get some more details 
>>>>> about the interaction with stfl(11) in the hypervisor.
>>>>>
>>>>
>>>> OK, may be we do not need to handle the case stfl(11) is not present 
>>>> in the host, these are pre GA10...
>>>
>>> What about VSIE? For all existing KVM guests, stfl11 is off.
>>
>> In VSIE the patch activates stfl(11) only if the host has stfl(11).
>>
>> I do not see any problem to activate the interpretation in VSIE with 
>> ECB_PTF (ECB.7) when the host has stfl(11) and QEMU asks to enable it 
>> for the guest using the CAPABILITY as it is done in this patch.
>>
>> if any intermediary hypervizor decide to not advertize stfl(11) for 
>> the guest like an old QEMU not having the CAPABILITY, or a QEMU with 
>> ctop=off, KVM will not set ECB_PTF and the PTF instruction will 
>> trigger a program check as before.
>>
>> Is it OK or did I missed something?
> 
> Yes, sure.
> My point was regarding the pre z10 statement.  We will see hosts without 
> stfl(e)11 when running nested on z14, z15 and co.

Ah OK, yes.
understood.

Thanks,
Pierre


-- 
Pierre Morel
IBM Lab Boeblingen