Re: [PATCH v12 3/3] KVM: s390: resetting the Topology-Change-Report

Janosch Frank
2022-07-13T09:02:11Z

On 7/12/22 13:17, Pierre Morel wrote:
>
>
> On 7/12/22 10:47, Janis Schoetterl-Glausch wrote:
>> On 7/12/22 09:24, Pierre Morel wrote:
>>>
>>>
>>> On 7/11/22 15:22, Janis Schoetterl-Glausch wrote:
>>>> On 7/11/22 10:41, Pierre Morel wrote:
>>>>> During a subsystem reset the Topology-Change-Report is cleared.
>>>>>
>>>>> Let's give userland the possibility to clear the MTCR in the case
>>>>> of a subsystem reset.
>>>>>
>>>>> To migrate the MTCR, we give userland the possibility to
>>>>> query the MTCR state.
>>>>>
>>>>> We indicate KVM support for the CPU topology facility with a new
>>>>> KVM capability: KVM_CAP_S390_CPU_TOPOLOGY.
>>>>>
>>>>> Signed-off-by: Pierre Morel <pmorel@linux.ibm.com>
>>>>
>>>> Reviewed-by: Janis Schoetterl-Glausch <scgl@linux.ibm.com>
>>>>
>>>
>>> Thanks!
>>>
>>>> See nits/comments below.
>>>>
>>>>> ---
>>>>>   Documentation/virt/kvm/api.rst   | 25 ++++++++++++++
>>>>>   arch/s390/include/uapi/asm/kvm.h |  1 +
>>>>>   arch/s390/kvm/kvm-s390.c         | 56 ++++++++++++++++++++++++++++++++
>>>>>   include/uapi/linux/kvm.h         |  1 +
>>>>>   4 files changed, 83 insertions(+)
>>>>>
>>>>> diff --git a/Documentation/virt/kvm/api.rst b/Documentation/virt/kvm/api.rst
>>>>> index 11e00a46c610..5e086125d8ad 100644
>>>>> --- a/Documentation/virt/kvm/api.rst
>>>>> +++ b/Documentation/virt/kvm/api.rst
>>>>> @@ -7956,6 +7956,31 @@ should adjust CPUID leaf 0xA to reflect that the PMU is disabled.
>>>>>   When enabled, KVM will exit to userspace with KVM_EXIT_SYSTEM_EVENT of
>>>>>   type KVM_SYSTEM_EVENT_SUSPEND to process the guest suspend request.
>>>>>   +8.37 KVM_CAP_S390_CPU_TOPOLOGY
>>>>> +------------------------------
>>>>> +
>>>>> +:Capability: KVM_CAP_S390_CPU_TOPOLOGY
>>>>> +:Architectures: s390
>>>>> +:Type: vm
>>>>> +
>>>>> +This capability indicates that KVM will provide the S390 CPU Topology
>>>>> +facility which consist of the interpretation of the PTF instruction for
>>>>> +the function code 2 along with interception and forwarding of both the
>>>>> +PTF instruction with function codes 0 or 1 and the STSI(15,1,x)
>>>>
>>>> Is the architecture allowed to extend STSI without a facility?
>>>> If so, if we say here that STSI 15.1.x is passed to user space, then
>>>> I think we should have a
>>>>
>>>> if (sel1 != 1)
>>>>     goto out_no_data;
>>>>
>>>> or maybe even
>>>>
>>>> if (sel1 != 1 || sel2 < 2 || sel2 > 6)
>>>>     goto out_no_data;
>>>>
>>>> in priv.c
>>>
>>> I am not a big fan of doing everything in the kernel.
>>> Here we have no performance issue since it is an error of the guest if it sends a wrong selector.
>>>
>> I agree, but I didn't suggest it for performance reasons.
>
> Yes, and that is why I do not agree ;)
>
>> I was thinking about future proofing, that is if the architecture is extended.
>> We don't know if future extensions are best handled in the kernel or user space,
>> so if we prevent it from going to user space, we can defer the decision to when we know more.
>
> If future extensions are better handle in kernel we will handle them in
> kernel, obviously, in this case we will need a patch.
>
> If it is not better handle in kernel we will handle the extensions in
> userland and we will not need a kernel patch making the update of the
> virtual architecture easier and faster.
>
> If we prohibit the extensions in kernel we will need a kernel patch in
> both cases and a userland patch if it is not completely handled in kernel.
>
> In userland we check any wrong selector before the instruction goes back
> to the guest.

I opt for passing the lower selectors down for QEMU to handle.

>
>> But that's only relevant if STSI can be extended without a capability, which is why I asked about that.
>
> Logicaly any change, extension, in the architecture should be signaled
> by a facility bit or something.
>
>>
>>> Even testing the facility or PV in the kernel is for my opinion arguable in the case we do not do any treatment in the kernel.

That's actually a good point.

New instruction interceptions for PV will need to be enabled by KVM via
a switch somewhere since the UV can't rely on the fact that KVM will
correctly handle it without an enablement.

So please remove the pv check

>>>
>>> I do not see what it brings to us, it increase the LOCs and makes the implementation less easy to evolve.
>>>
>>>
>>>>
>>>>> +instruction to the userland hypervisor.
>>>>> +
>>>>> +The stfle facility 11, CPU Topology facility, should not be indicated
>>>>> +to the guest without this capability.
>>>>> +
>>>>> +When this capability is present, KVM provides a new attribute group
>>>>> +on vm fd, KVM_S390_VM_CPU_TOPOLOGY.
>>>>> +This new attribute allows to get, set or clear the Modified Change
>>>>
>>>> get or set, now that there is no explicit clear anymore.
>>>
>>> Yes now it is a set to 0 but the action of clearing remains.

Yes

>>>
>>>>
>>>>> +Topology Report (MTCR) bit of the SCA through the kvm_device_attr
>>>>> +structure.> +
>>>>> +When getting the Modified Change Topology Report value, the attr->addr
>>>>
>>>> When getting/setting the...
>>>>
>>>>> +must point to a byte where the value will be stored.
>>>>
>>>> ... will be stored/retrieved from.
>>>
>>> OK
>>
>> Wait no, I didn't get how that works. You're passing the value via attr->attr, not reading it from addr.
>
> :) OK
>
>>>
>>>
>>>>> +
>>>>>   9. Known KVM API problems
>>>>>   =========================
>>>>>   diff --git a/arch/s390/include/uapi/asm/kvm.h b/arch/s390/include/uapi/asm/kvm.h
>>>>> index 7a6b14874d65..a73cf01a1606 100644
>>>>> --- a/arch/s390/include/uapi/asm/kvm.h
>>>>> +++ b/arch/s390/include/uapi/asm/kvm.h
>>>>> @@ -74,6 +74,7 @@ struct kvm_s390_io_adapter_req {
>>>>>   #define KVM_S390_VM_CRYPTO        2
>>>>>   #define KVM_S390_VM_CPU_MODEL        3
>>>>>   #define KVM_S390_VM_MIGRATION        4
>>>>> +#define KVM_S390_VM_CPU_TOPOLOGY    5
>>>>>     /* kvm attributes for mem_ctrl */
>>>>>   #define KVM_S390_VM_MEM_ENABLE_CMMA    0
>>>>> diff --git a/arch/s390/kvm/kvm-s390.c b/arch/s390/kvm/kvm-s390.c
>>>>> index 70436bfff53a..b18e0b940b26 100644
>>>>> --- a/arch/s390/kvm/kvm-s390.c
>>>>> +++ b/arch/s390/kvm/kvm-s390.c
>>>>> @@ -606,6 +606,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
>>>>>       case KVM_CAP_S390_PROTECTED:
>>>>>           r = is_prot_virt_host();
>>>>>           break;
>>>>> +    case KVM_CAP_S390_CPU_TOPOLOGY:
>>>>> +        r = test_facility(11);
>>>>> +        break;
>>>>>       default:
>>>>>           r = 0;
>>>>>       }
>>>>> @@ -817,6 +820,20 @@ 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:
>>>>> +        r = -EINVAL;
>>>>> +        mutex_lock(&kvm->lock);
>>>>> +        if (kvm->created_vcpus) {
>>>>> +            r = -EBUSY;
>>>>> +        } 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;
>>>>> +        }
>>>>> +        mutex_unlock(&kvm->lock);
>>>>> +        VM_EVENT(kvm, 3, "ENABLE: CAP_S390_CPU_TOPOLOGY %s",
>>>>> +             r ? "(not available)" : "(success)");
>>>>> +        break;
>>>>>       default:
>>>>>           r = -EINVAL;
>>>>>           break;
>>>>> @@ -1717,6 +1734,36 @@ static void kvm_s390_update_topology_change_report(struct kvm *kvm, bool val)
>>>>>       read_unlock(&kvm->arch.sca_lock);
>>>>>   }
>>>>>   +static int kvm_s390_set_topology(struct kvm *kvm, struct kvm_device_attr *attr)
>>>>
>>>> kvm_s390_set_topology_changed maybe?
>>>> kvm_s390_get_topology_changed below then.

kvm_s390_set_topology_change_indication

It's long but it's rarely used.
Maybe shorten topology to "topo"

[..]
>>>> I don't think you need the READ_ONCE anymore, now that there is a lock it should act as a compile barrier.
>>>
>>> I think you are right.
>>>
>>>>> +    read_unlock(&kvm->arch.sca_lock);
>>>>> +    topo = utility.mtcr;
>>>>> +
>>>>> +    if (copy_to_user((void __user *)attr->addr, &topo, sizeof(topo)))
>>>>
>>>> Why void not u8?
>>>
>>> I like to say we write on "topo" with the size of "topo".
>>> So we do not need to verify the effective size of topo.
>>> But I understand, it is a UAPI, setting u8 in the copy_to_user makes sense too.
>>> For my personal opinion, I would have prefer that userland tell us the size it awaits even here, for this special case, since we use a byte, we can not do really wrong.
>> You're right, it doesn't make a difference.
>> What about doing put_user(topo, (u8 *)attr->addr)), seems more straight forward.
>
> OK

(u8 __user *)

Always go the explicit route if possible

>
>>>
>>>>
>>>>> +        return -EFAULT;
>>>>> +
>>>>> +    return 0;
>>>>> +}
>>>>> +
>>>> [...]
>>>>
>>>
>>
>