Re: [PATCH v3 RESEND 04/10] KVM: PPC: selftests: powerpc enable kvm_create_max_vcpus test

[Ritesh Harjani (IBM) <ritesh.list@gmail.com>]

Sean Christopherson <seanjc@google.com> writes:

> On Fri, Jun 12, 2026, Ritesh Harjani wrote:
>> Sean Christopherson <seanjc@google.com> writes:
>> 
>> > On Wed, Jun 10, 2026, Ritesh Harjani (IBM) wrote:
>> >> From: Nicholas Piggin <npiggin@gmail.com>
>> >> 
>> >> powerpc's maximum permitted vCPU ID depends on the VM's SMT mode, and
>> >> the maximum reported by KVM_CAP_MAX_VCPU_ID exceeds a simple non-SMT
>> >> VM's limit.
>> >> 
>> >> The powerpc KVM selftest port uses non-SMT VMs, so add a workaround
>> >> to the kvm_create_max_vcpus test case to limit vCPU IDs to
>> >> KVM_CAP_MAX_VCPUS on powerpc.
>> >
>> > How is this not a KVM bug?  Literally the reason this test exists is to validate
>> > KVM's advertised KVM_CAP_MAX_VCPU_ID and KVM_CAP_MAX_VCPUS.
>> 
>> It's not a KVM bug, it's expected on PowerPC. On PowerPC, vCPU ID encodes SMT topology, e.g. on P9,
>> vcpu id = core * stride + thread,
>>   .. where the stride is same as kvm->arch.emul_smt_mode (VM's emulated SMT mode)
>> 
>> So the vcpu ID space can be sparse, however KVM_CAP_MAX_VCPU_ID is the
>> absolute ceil value (MAX_SMT_THREADS * KVM_MAX_VCORES) i.e. the value
>> with the maximum stride / SMT value.
>> 
>> Since default selftest VM uses stride 1, so it rejects IDS >= max_vcpus.
>> 
>> e.g.
>> static int kvmppc_core_vcpu_create_hv(struct kvm_vcpu *vcpu)
>> ...
>> 		if (id >= (KVM_MAX_VCPUS * kvm->arch.emul_smt_mode)) {
>> 			pr_devel("KVM: VCPU ID too high\n");
>> 			core = KVM_MAX_VCORES;              /* rejected case */
>> 		} else {
>> 
>> So, it's expected on PowerPC. vcpus with higher IDs can be created but
>> for that we need to set KVM_CAP_PPC_SMT and use strided (sparse) IDs.
>> But since the test as of now is not doing that - that's the reason why
>> the patch only allows to test max vcpu IDs upto max vcpus.
>> 
>> But I guess you must be hating the #ifdef __powerpc__ there. I agree I
>> don't like it either.. maybe we can do it this way?
>
> I don't love the #ifdef, but it's more that I didn't want to effectively skip a
> test because KVM was reporting bad information.  But after peeking at KVM_CAP_PPC_SMT,
> I 100% agree we don't want to deal with that here.
>
>> -#ifdef __powerpc64__
>>         /*
>> -        * powerpc has a particular format for the vcpu ID that depends on
>> -        * the guest SMT mode, and the max ID cap is too large for non-SMT
>> -        * modes, where the maximum ID is the same as the maximum vCPUs.
>> +        * Some architectures (e.g. powerpc) encode topology into the vCPU ID,
>> +        * so a default VM can't necessarily use the full advertised ID range.
>> +        * Let the arch limit the highest ID this test will create.
>>          */
>> -       kvm_max_vcpu_id = kvm_max_vcpus;
>> -#endif
>> +       kvm_max_vcpu_id = kvm_arch_vcpu_id_limit(kvm_max_vcpus, kvm_max_vcpu_id);
>> 
>> 
>> And then in kvm_util.c -
>> +
>> +__weak int kvm_arch_vcpu_id_limit(int nr_vcpus, int vcpu_id_max)
>> +{
>> +       return vcpu_id_max;
>> +}
>
> What if we do this?  We're going to be bleeding PPC details into the test no
> matter what, adding an arch hooks just seems like extra cruft and an unnecessary
> layer of indirection.
>
> 	/*
> 	 * Skip the vCPU ID test when running on PowerPC with SMT support, in
> 	 * which case system topology is encoded into the vCPU ID, and so a VM
> 	 * can't use the full advertised vCPU ID range without crafting a valid
> 	 * platform specific topology.
> 	 */
> 	if (kvm_max_vcpu_id > kvm_max_vcpus && !kvm_check_cap(KVM_CAP_PPC_SMT))
> 		test_vcpu_creation(
> 			kvm_max_vcpu_id - kvm_max_vcpus, kvm_max_vcpus);

Thanks! That should do and I think that's the most cleaner version.

I will change patch-4 with your suggested changes then.

-ritesh