From mboxrd@z Thu Jan 1 00:00:00 1970 From: Christophe Leroy Date: Tue, 08 Mar 2022 13:34:09 +0000 Subject: Re: [RFC PATCH 2/2] KVM: PPC: Book3S HV: Provide a more accurate MAX_VCPU_ID in P9 Message-Id: List-Id: References: <20210412222656.3466987-1-farosas@linux.ibm.com> <20210412222656.3466987-3-farosas@linux.ibm.com> In-Reply-To: <20210412222656.3466987-3-farosas@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset="utf-8" Content-Transfer-Encoding: 8bit To: Fabiano Rosas , kvm-ppc@vger.kernel.org Cc: linuxppc-dev@lists.ozlabs.org, groug@kaod.org, david@gibson.dropbear.id.au 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 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. Christsophe > > --- > I see that for ppc, QEMU uses the capability after enabling > KVM_CAP_PPC_SMT, so we could change QEMU to issue the check extension > on the vm fd so that it would get the more accurate value. > --- > arch/powerpc/kvm/powerpc.c | 5 ++++- > 1 file changed, 4 insertions(+), 1 deletion(-) > > diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c > index a2a68a958fa0..95c9f47cc1b3 100644 > --- a/arch/powerpc/kvm/powerpc.c > +++ b/arch/powerpc/kvm/powerpc.c > @@ -649,7 +649,10 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext) > r = KVM_MAX_VCPUS; > break; > case KVM_CAP_MAX_VCPU_ID: > - r = KVM_MAX_VCPU_ID; > + if (hv_enabled && cpu_has_feature(CPU_FTR_ARCH_300)) > + r = KVM_MAX_VCPUS * ((kvm) ? kvm->arch.emul_smt_mode : 1); > + else > + r = KVM_MAX_VCPU_ID; > break; > #ifdef CONFIG_PPC_BOOK3S_64 > case KVM_CAP_PPC_GET_SMMU_INFO: