linux-kernel - Re: [PATCH v2 2/2] s390:kvm: Topology expose TOPOLOGY facility

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Message-ID: <68c2e0d0-b591-7701-700c-400f1f040ca9@linux.ibm.com>
Date:   Fri, 23 Jul 2021 12:10:03 +0200
From:   Pierre Morel <pmorel@...ux.ibm.com>
To:     Christian Borntraeger <borntraeger@...ibm.com>,
        Cornelia Huck <cohuck@...hat.com>, kvm@...r.kernel.org
Cc:     linux-s390@...r.kernel.org, linux-kernel@...r.kernel.org,
        frankja@...ux.ibm.com, david@...hat.com, thuth@...hat.com,
        imbrenda@...ux.ibm.com, hca@...ux.ibm.com, gor@...ux.ibm.com
Subject: Re: [PATCH v2 2/2] s390:kvm: Topology expose TOPOLOGY facility



On 7/23/21 11:28 AM, Christian Borntraeger wrote:
> 
> 
> On 23.07.21 10:55, Cornelia Huck wrote:
>> On Fri, Jul 23 2021, Christian Borntraeger <borntraeger@...ibm.com> 
>> wrote:
>>
>>> On 22.07.21 19:02, Pierre Morel wrote:
>>>> We add a KVM extension KVM_CAP_S390_CPU_TOPOLOGY to tell the
>>>> userland hypervisor it is safe to activate the CPU Topology facility.
>>>
>>> I think the old variant of using the CPU model was actually better.
>>> It was just the patch description that was wrong.
>>
>> I thought we wanted a cap that userspace can enable to get ptf
>> intercepts? I'm confused.
>>
> 
> PTF goes to userspace in any case as every instruction that is
> not handled by kvm and where interpretion is not enabled.
> Now, having said that, we actually want PTF interpretion to be enabled
> for "Check topology-change status" as this is supposed to be a fast
> operation. Some OSes do query that in their interrupt handlers.
> 

An old QEMU getting the PTF instruction will send a OPERATION exception 
to the guest if the facility 11 is actzivated.
Facility 11 is in QEMU since GAEN10_GA1, if I enable the facility in the 
CPU model all cpu model starting with GEN10_GA1 will panic on PTF.

So I think we need the capability so that new QEMU enable the facility 
once it has the right handling for PTF.


> 
>>>> Signed-off-by: Pierre Morel <pmorel@...ux.ibm.com>
>>>> ---
>>>>    arch/s390/kvm/kvm-s390.c | 1 +
>>>>    include/uapi/linux/kvm.h | 1 +
>>>>    2 files changed, 2 insertions(+)
>>>>
>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>> index b655a7d82bf0..8c695ee79612 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:
>>>>            r = 1;
>>>>            break;
>>>>        case KVM_CAP_SET_GUEST_DEBUG2:
>>>> 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
>>>>    #ifdef KVM_CAP_IRQ_ROUTING
>>>>
>>
>> Regardless of what we end up with: we need documentation for any new cap
>> :)
>>

-- 
Pierre Morel
IBM Lab Boeblingen