Subject: Re: [RFC PATCH 2/2] KVM: PPC: Book3S HV: Provide a more accurate MAX_VCPU_ID in P9

From: Fabiano Rosas <[email-masked]>
Date: Tue, 08 Mar 2022 11:27:32 -0300

Christophe Leroy <[email-masked]> writes:

> Le 13/04/2021 à 00:26, Fabiano Rosas a écrit :
>> The KVM_CAP_MAX_VCPU_ID capability was added by commit 0b1b1dfd52a6
>> ("kvm: introduce KVM_MAX_VCPU_ID") to allow for vcpu ids larger than
>> KVM_MAX_VCPU in powerpc.
>> 
>> For a P9 host we depend on the guest VSMT value to know what is the
>> maximum number of vcpu id we can support:
>> 
>> kvmppc_core_vcpu_create_hv:
>>      (...)
>>      if (cpu_has_feature(CPU_FTR_ARCH_300)) {
>> -->         if (id >= (KVM_MAX_VCPUS * kvm->arch.emul_smt_mode)) {
>>                      pr_devel("KVM: VCPU ID too high\n");
>>                      core = KVM_MAX_VCORES;
>>              } else {
>>                      BUG_ON(kvm->arch.smt_mode != 1);
>>                      core = kvmppc_pack_vcpu_id(kvm, id);
>>              }
>>      } else {
>>              core = id / kvm->arch.smt_mode;
>>      }
>> 
>> which means that the value being returned by the capability today for
>> a given guest is potentially way larger than what we actually support:
>> 
>> \#define KVM_MAX_VCPU_ID (MAX_SMT_THREADS * KVM_MAX_VCORES)
>> 
>> If the capability is queried before userspace enables the
>> KVM_CAP_PPC_SMT ioctl there is not much we can do, but if the emulated
>> smt mode is already known we could provide a more accurate value.
>> 
>> The only practical effect of this change today is to make the
>> kvm_create_max_vcpus test pass for powerpc. The QEMU spapr machine has
>> a lower max vcpu than what KVM allows so even KVM_MAX_VCPU is not
>> reached.
>> 
>> Signed-off-by: Fabiano Rosas <[email-masked]>
>
> This patch won't apply after commit a1c42ddedf35 ("kvm: rename 
> KVM_MAX_VCPU_ID to KVM_MAX_VCPU_IDS")
>
> Feel free to resubmit if still applicable.

Thanks for the reminder, Christophe.

I was focusing on enabling the rest of the kvm-selftests:
https://lore.kernel.org/r/[email-masked]

I'm preparing a v2 for that series and will try to include these patches
as well.