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From: Fabiano Rosas <farosas@linux.ibm.com>
To: Christophe Leroy <christophe.leroy@csgroup.eu>, kvm-ppc@vger.kernel.org
Subject: Re: [RFC PATCH 2/2] KVM: PPC: Book3S HV: Provide a more accurate
 MAX_VCPU_ID in P9
In-Reply-To: <f5ec9d55-bac3-b571-dfad-bd501cd364b3@csgroup.eu>
References: <20210412222656.3466987-1-farosas@linux.ibm.com>
 <20210412222656.3466987-3-farosas@linux.ibm.com>
 <f5ec9d55-bac3-b571-dfad-bd501cd364b3@csgroup.eu>
Date: Tue, 08 Mar 2022 11:27:32 -0300
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 <linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org>

Christophe Leroy <christophe.leroy@csgroup.eu> writes:

> Le 13/04/2021 =C3=A0 00:26, Fabiano Rosas a =C3=A9crit=C2=A0:
>> 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.
>>=20
>> For a P9 host we depend on the guest VSMT value to know what is the
>> maximum number of vcpu id we can support:
>>=20
>> kvmppc_core_vcpu_create_hv:
>>      (...)
>>      if (cpu_has_feature(CPU_FTR_ARCH_300)) {
>> -->         if (id >=3D (KVM_MAX_VCPUS * kvm->arch.emul_smt_mode)) {
>>                      pr_devel("KVM: VCPU ID too high\n");
>>                      core =3D KVM_MAX_VCORES;
>>              } else {
>>                      BUG_ON(kvm->arch.smt_mode !=3D 1);
>>                      core =3D kvmppc_pack_vcpu_id(kvm, id);
>>              }
>>      } else {
>>              core =3D id / kvm->arch.smt_mode;
>>      }
>>=20
>> which means that the value being returned by the capability today for
>> a given guest is potentially way larger than what we actually support:
>>=20
>> \#define KVM_MAX_VCPU_ID (MAX_SMT_THREADS * KVM_MAX_VCORES)
>>=20
>> 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.
>>=20
>> 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.
>>=20
>> Signed-off-by: Fabiano Rosas <farosas@linux.ibm.com>
>
> This patch won't apply after commit a1c42ddedf35 ("kvm: rename=20
> 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/20220120170109.948681-1-farosas@linux.ibm.com

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