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

[Christophe Leroy <christophe.leroy@csgroup.eu>]

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 <farosas@linux.ibm.com>

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: