KVM: PPC: Book3S HV: Add capability to report possible virtual SMT modes

Now that userspace can set the virtual SMT mode by enabling the
KVM_CAP_PPC_SMT capability, it is useful for userspace to be able
to query the set of possible virtual SMT modes.  This provides a
new capability, KVM_CAP_PPC_SMT_POSSIBLE, to provide this
information.  The return value is a bitmap of possible modes, with
bit N set if virtual SMT mode 2^N is available.  That is, 1 indicates
SMT1 is available, 2 indicates that SMT2 is available, 3 indicates
that both SMT1 and SMT2 are available, and so on.

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

3 files changed, 22 insertions, 0 deletions

diff --git a/Documentation/virtual/kvm/api.txt b/Documentation/virtual/kvm/api.txt
index 1531a3cd548f..63875dbb6ef5 100644
--- a/Documentation/virtual/kvm/api.txt
+++ b/Documentation/virtual/kvm/api.txt
@@ -4010,6 +4010,8 @@ be 0.  A successful call to enable this capability will result in
 vsmt_mode being returned when the KVM_CAP_PPC_SMT capability is
 subsequently queried for the VM.  This capability is only supported by
 HV KVM, and can only be set before any VCPUs have been created.
+The KVM_CAP_PPC_SMT_POSSIBLE capability indicates which virtual SMT
+modes are available.
 
 7.12 KVM_CAP_PPC_FWNMI
 
@@ -4183,3 +4185,12 @@ Currently the following bits are defined for the device_irq_level bitmap:
 Future versions of kvm may implement additional events. These will get
 indicated by returning a higher number from KVM_CHECK_EXTENSION and will be
 listed above.
+
+8.10 KVM_CAP_PPC_SMT_POSSIBLE
+
+Architectures: ppc
+
+Querying this capability returns a bitmap indicating the possible
+virtual SMT modes that can be set using KVM_CAP_PPC_SMT.  If bit N
+(counting from the right) is set, then a virtual SMT mode of 2^N is
+available.
diff --git a/arch/powerpc/kvm/powerpc.c b/arch/powerpc/kvm/powerpc.c
index ccaa7a407c15..b14736f3870b 100644
--- a/arch/powerpc/kvm/powerpc.c
+++ b/arch/powerpc/kvm/powerpc.c
@@ -566,6 +566,16 @@ int kvm_vm_ioctl_check_extension(struct kvm *kvm, long ext)
                                 r = threads_per_subcore;
                 }
                 break;
+        case KVM_CAP_PPC_SMT_POSSIBLE:
+                r = 1;
+                if (hv_enabled) {
+                        if (!cpu_has_feature(CPU_FTR_ARCH_300))
+                                r = ((threads_per_subcore << 1) - 1);
+                        else
+                                /* P9 can emulate dbells, so allow any mode */
+                                r = 8 | 4 | 2 | 1;
+                }
+                break;
         case KVM_CAP_PPC_RMA:
                 r = 0;
                 break;
diff --git a/include/uapi/linux/kvm.h b/include/uapi/linux/kvm.h
index 89bc145b4051..36ea74aa3422 100644
--- a/include/uapi/linux/kvm.h
+++ b/include/uapi/linux/kvm.h
@@ -896,6 +896,7 @@ struct kvm_ppc_resize_hpt {
 #define KVM_CAP_X86_GUEST_MWAIT 143
 #define KVM_CAP_ARM_USER_IRQ 144
 #define KVM_CAP_PPC_FWNMI 145
+#define KVM_CAP_PPC_SMT_POSSIBLE 146
 
 #ifdef KVM_CAP_IRQ_ROUTING