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

    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.

    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.

    >
    >> +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


    >> +
    >> 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.

    No strong opinion, if you prefer I change this.

    >
    >> +{
    >> + if (!test_kvm_facility(kvm, 11))
    >> + return -ENXIO;
    >> +
    >> + kvm_s390_update_topology_change_report(kvm, !!attr->attr);
    >> + return 0;
    >> +}
    >> +
    >> +static int kvm_s390_get_topology(struct kvm *kvm, struct kvm_device_attr *attr)
    >> +{
    >> + union sca_utility utility;
    >> + struct bsca_block *sca;
    >> + __u8 topo;
    >> +
    >> + if (!test_kvm_facility(kvm, 11))
    >> + return -ENXIO;
    >> +
    >> + read_lock(&kvm->arch.sca_lock);
    >> + sca = kvm->arch.sca;
    >> + utility.val = READ_ONCE(sca->utility.val);
    >
    > 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.

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

    --
    Pierre Morel
    IBM Lab Boeblingen

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