KVM: PPC: Book3S HV: Allow userspace to set the desired SMT mode

This allows userspace to set the desired virtual SMT (simultaneous
multithreading) mode for a VM, that is, the number of VCPUs that
get assigned to each virtual core.  Previously, the virtual SMT mode
was fixed to the number of threads per subcore, and if userspace
wanted to have fewer vcpus per vcore, then it would achieve that by
using a sparse CPU numbering.  This had the disadvantage that the
vcpu numbers can get quite large, particularly for SMT1 guests on
a POWER8 with 8 threads per core.  With this patch, userspace can
set its desired virtual SMT mode and then use contiguous vcpu
numbering.

On POWER8, where the threading mode is "strict", the virtual SMT mode
must be less than or equal to the number of threads per subcore.  On
POWER9, which implements a "loose" threading mode, the virtual SMT
mode can be any power of 2 between 1 and 8, even though there is
effectively one thread per subcore, since the threads are independent
and can all be in different partitions.

Signed-off-by: Paul Mackerras <paulus@ozlabs.org>

1 files changed, 12 insertions, 1 deletions

diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index 7f71ab5fcad1..da90ae8cd508 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -554,7 +554,9 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
 #ifdef CONFIG_KVM_BOOK3S_HV_POSSIBLE
 	case KVM_CAP_PPC_SMT:
 		r = 0;
-		if (hv_enabled) {
+		if (kvm)
+			r = kvm->arch.smt_mode;
+		else if (hv_enabled) {
 			if (cpu_has_feature(CPU_FTR_ARCH_300))
 				r = 1;
 			else
@@ -1712,6 +1714,15 @@ static int kvm_vm_ioctl_enable_cap(struct kvm *kvm,
 		r = 0;
 		break;
 	}
+	case KVM_CAP_PPC_SMT: {
+		unsigned long mode = cap->args[0];
+		unsigned long flags = cap->args[1];
+
+		r = -EINVAL;
+		if (kvm->arch.kvm_ops->set_smt_mode)
+			r = kvm->arch.kvm_ops->set_smt_mode(kvm, mode, flags);
+		break;
+	}
 #endif
 	default:
 		r = -EINVAL;