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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,
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From:   Pierre Morel <pmorel@linux.ibm.com>
Message-ID: <316097be-81f6-4667-0245-a32d62b19a25@linux.ibm.com>
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