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Message-ID: <98a3de32-0ac5-00f3-8799-dfff8edae70d@linux.ibm.com>
Date: Wed, 8 Sep 2021 16:17:22 +0200
From: Pierre Morel <pmorel@...ux.ibm.com>
To: Christian Borntraeger <borntraeger@...ibm.com>,
David Hildenbrand <david@...hat.com>, kvm@...r.kernel.org
Cc: linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
frankja@...ux.ibm.com, cohuck@...hat.com, thuth@...hat.com,
imbrenda@...ux.ibm.com, hca@...ux.ibm.com, gor@...ux.ibm.com
Subject: Re: [PATCH v3 2/3] s390x: KVM: Implementation of Multiprocessor
Topology-Change-Report
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@...ux.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
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