PPC: KVM: Support machine option to set VSMT mode

KVM now allows writing to KVM_CAP_PPC_SMT which has previously been
read only. Doing so causes KVM to act, for that VM, as if the host's
SMT mode was the given value. This is particularly important on Power
9 systems because their default value is 1, but they are able to
support values up to 8.

This patch introduces a way to control this capability via a new
machine property called VSMT ("Virtual SMT"). If the value is not set
on the command line a default is chosen that is, when possible,
compatible with legacy systems.

Note that the intialization of KVM_CAP_PPC_SMT has changed slightly
because it has changed (in KVM) from a global capability to a
VM-specific one. This won't cause a problem on older KVMs because VM
capabilities fall back to global ones.

Signed-off-by: Sam Bobroff <sam.bobroff@au1.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>

1 files changed, 75 insertions, 0 deletions

diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index 067f571..caffa12 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -26,6 +26,7 @@
  */
 #include "qemu/osdep.h"
 #include "qapi/error.h"
+#include "qapi/visitor.h"
 #include "sysemu/sysemu.h"
 #include "sysemu/numa.h"
 #include "hw/hw.h"
@@ -2165,6 +2166,61 @@ static void spapr_init_cpus(sPAPRMachineState *spapr)
     g_free(type);
 }
 
+static void spapr_set_vsmt_mode(sPAPRMachineState *spapr, Error **errp)
+{
+    Error *local_err = NULL;
+    bool vsmt_user = !!spapr->vsmt;
+    int kvm_smt = kvmppc_smt_threads();
+    int ret;
+
+    if (!kvm_enabled() && (smp_threads > 1)) {
+        error_setg(&local_err, "TCG cannot support more than 1 thread/core "
+                     "on a pseries machine");
+        goto out;
+    }
+    if (!is_power_of_2(smp_threads)) {
+        error_setg(&local_err, "Cannot support %d threads/core on a pseries "
+                     "machine because it must be a power of 2", smp_threads);
+        goto out;
+    }
+
+    /* Detemine the VSMT mode to use: */
+    if (vsmt_user) {
+        if (spapr->vsmt < smp_threads) {
+            error_setg(&local_err, "Cannot support VSMT mode %d"
+                         " because it must be >= threads/core (%d)",
+                         spapr->vsmt, smp_threads);
+            goto out;
+        }
+        /* In this case, spapr->vsmt has been set by the command line */
+    } else {
+        /* Choose a VSMT mode that may be higher than necessary but is
+         * likely to be compatible with hosts that don't have VSMT. */
+        spapr->vsmt = MAX(kvm_smt, smp_threads);
+    }
+
+    /* KVM: If necessary, set the SMT mode: */
+    if (kvm_enabled() && (spapr->vsmt != kvm_smt)) {
+        ret = kvmppc_set_smt_threads(spapr->vsmt);
+        if (ret) {
+            error_setg(&local_err,
+                       "Failed to set KVM's VSMT mode to %d (errno %d)",
+                       spapr->vsmt, ret);
+            if (!vsmt_user) {
+                error_append_hint(&local_err, "On PPC, a VM with %d threads/"
+                             "core on a host with %d threads/core requires "
+                             " the use of VSMT mode %d.\n",
+                             smp_threads, kvm_smt, spapr->vsmt);
+            }
+            kvmppc_hint_smt_possible(&local_err);
+            goto out;
+        }
+    }
+    /* else TCG: nothing to do currently */
+out:
+    error_propagate(errp, local_err);
+}
+
 /* pSeries LPAR / sPAPR hardware init */
 static void ppc_spapr_init(MachineState *machine)
 {
@@ -2297,6 +2353,8 @@ static void ppc_spapr_init(MachineState *machine)
 
     spapr_cpu_parse_features(spapr);
 
+    spapr_set_vsmt_mode(spapr, &error_fatal);
+
     spapr_init_cpus(spapr);
 
     if (kvm_enabled()) {
@@ -2681,6 +2739,18 @@ static void spapr_set_resize_hpt(Object *obj, const char *value, Error **errp)
     }
 }
 
+static void spapr_get_vsmt(Object *obj, Visitor *v, const char *name,
+                                   void *opaque, Error **errp)
+{
+    visit_type_uint32(v, name, (uint32_t *)opaque, errp);
+}
+
+static void spapr_set_vsmt(Object *obj, Visitor *v, const char *name,
+                                   void *opaque, Error **errp)
+{
+    visit_type_uint32(v, name, (uint32_t *)opaque, errp);
+}
+
 static void spapr_machine_initfn(Object *obj)
 {
     sPAPRMachineState *spapr = SPAPR_MACHINE(obj);
@@ -2711,6 +2781,11 @@ static void spapr_machine_initfn(Object *obj)
     object_property_set_description(obj, "resize-hpt",
                                     "Resizing of the Hash Page Table (enabled, disabled, required)",
                                     NULL);
+    object_property_add(obj, "vsmt", "uint32", spapr_get_vsmt,
+                        spapr_set_vsmt, NULL, &spapr->vsmt, &error_abort);
+    object_property_set_description(obj, "vsmt",
+                                    "Virtual SMT: KVM behaves as if this were"
+                                    " the host's SMT mode", &error_abort);
 }
 
 static void spapr_machine_finalizefn(Object *obj)