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

[Fabiano Rosas]

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 at linux.ibm.com>

---
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:
-- 
2.29.2