machine: Prefer cores over sockets in smp parsing since 6.2

In the real SMP hardware topology world, it's much more likely that
we have high cores-per-socket counts and few sockets totally. While
the current preference of sockets over cores in smp parsing results
in a virtual cpu topology with low cores-per-sockets counts and a
large number of sockets, which is just contrary to the real world.

Given that it is better to make the virtual cpu topology be more
reflective of the real world and also for the sake of compatibility,
we start to prefer cores over sockets over threads in smp parsing
since machine type 6.2 for different arches.

In this patch, a boolean "smp_prefer_sockets" is added, and we only
enable the old preference on older machines and enable the new one
since type 6.2 for all arches by using the machine compat mechanism.

Suggested-by: Daniel P. Berrange <berrange@redhat.com>
Signed-off-by: Yanan Wang <wangyanan55@huawei.com>
Acked-by: David Gibson <david@gibson.dropbear.id.au>
Acked-by: Cornelia Huck <cohuck@redhat.com>
Reviewed-by: Andrew Jones <drjones@redhat.com>
Reviewed-by: Pankaj Gupta <pankaj.gupta@ionos.com>
Reviewed-by: Daniel P. Berrangé <berrange@redhat.com>
Message-Id: <20210929025816.21076-10-wangyanan55@huawei.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

7 files changed, 57 insertions, 18 deletions

diff --git a/hw/arm/virt.c b/hw/arm/virt.c
index 1d59f0e..8c13deb 100644
--- a/hw/arm/virt.c
+++ b/hw/arm/virt.c
@@ -2815,6 +2815,7 @@ static void virt_machine_6_1_options(MachineClass *mc)
 
     virt_machine_6_2_options(mc);
     compat_props_add(mc->compat_props, hw_compat_6_1, hw_compat_6_1_len);
+    mc->smp_prefer_sockets = true;
 
     /* qemu ITS was introduced with 6.2 */
     vmc->no_tcg_its = true;
diff --git a/hw/core/machine.c b/hw/core/machine.c
index e38ab76..f2a34d9 100644
--- a/hw/core/machine.c
+++ b/hw/core/machine.c
@@ -748,6 +748,7 @@ void machine_set_cpu_numa_node(MachineState *machine,
 
 static void smp_parse(MachineState *ms, SMPConfiguration *config, Error **errp)
 {
+    MachineClass *mc = MACHINE_GET_CLASS(ms);
     unsigned cpus    = config->has_cpus ? config->cpus : 0;
     unsigned sockets = config->has_sockets ? config->sockets : 0;
     unsigned cores   = config->has_cores ? config->cores : 0;
@@ -759,7 +760,7 @@ static void smp_parse(MachineState *ms, SMPConfiguration *config, Error **errp)
         return;
     }
 
-    /* compute missing values, prefer sockets over cores over threads */
+    /* compute missing values based on the provided ones */
     if (cpus == 0 && maxcpus == 0) {
         sockets = sockets > 0 ? sockets : 1;
         cores = cores > 0 ? cores : 1;
@@ -767,14 +768,30 @@ static void smp_parse(MachineState *ms, SMPConfiguration *config, Error **errp)
     } else {
         maxcpus = maxcpus > 0 ? maxcpus : cpus;
 
-        if (sockets == 0) {
-            cores = cores > 0 ? cores : 1;
-            threads = threads > 0 ? threads : 1;
-            sockets = maxcpus / (cores * threads);
-        } else if (cores == 0) {
-            threads = threads > 0 ? threads : 1;
-            cores = maxcpus / (sockets * threads);
-        } else if (threads == 0) {
+        if (mc->smp_prefer_sockets) {
+            /* prefer sockets over cores before 6.2 */
+            if (sockets == 0) {
+                cores = cores > 0 ? cores : 1;
+                threads = threads > 0 ? threads : 1;
+                sockets = maxcpus / (cores * threads);
+            } else if (cores == 0) {
+                threads = threads > 0 ? threads : 1;
+                cores = maxcpus / (sockets * threads);
+            }
+        } else {
+            /* prefer cores over sockets since 6.2 */
+            if (cores == 0) {
+                sockets = sockets > 0 ? sockets : 1;
+                threads = threads > 0 ? threads : 1;
+                cores = maxcpus / (sockets * threads);
+            } else if (sockets == 0) {
+                threads = threads > 0 ? threads : 1;
+                sockets = maxcpus / (cores * threads);
+            }
+        }
+
+        /* try to calculate omitted threads at last */
+        if (threads == 0) {
             threads = maxcpus / (sockets * cores);
         }
     }
diff --git a/hw/i386/pc.c b/hw/i386/pc.c
index 9216ad1..6cc32f4 100644
--- a/hw/i386/pc.c
+++ b/hw/i386/pc.c
@@ -716,6 +716,7 @@ void pc_acpi_smi_interrupt(void *opaque, int irq, int level)
  */
 static void pc_smp_parse(MachineState *ms, SMPConfiguration *config, Error **errp)
 {
+    MachineClass *mc = MACHINE_GET_CLASS(ms);
     unsigned cpus    = config->has_cpus ? config->cpus : 0;
     unsigned sockets = config->has_sockets ? config->sockets : 0;
     unsigned dies    = config->has_dies ? config->dies : 0;
@@ -726,7 +727,7 @@ static void pc_smp_parse(MachineState *ms, SMPConfiguration *config, Error **err
     /* directly default dies to 1 if it's omitted */
     dies = dies > 0 ? dies : 1;
 
-    /* compute missing values, prefer sockets over cores over threads */
+    /* compute missing values based on the provided ones */
     if (cpus == 0 && maxcpus == 0) {
         sockets = sockets > 0 ? sockets : 1;
         cores = cores > 0 ? cores : 1;
@@ -734,14 +735,30 @@ static void pc_smp_parse(MachineState *ms, SMPConfiguration *config, Error **err
     } else {
         maxcpus = maxcpus > 0 ? maxcpus : cpus;
 
-        if (sockets == 0) {
-            cores = cores > 0 ? cores : 1;
-            threads = threads > 0 ? threads : 1;
-            sockets = maxcpus / (dies * cores * threads);
-        } else if (cores == 0) {
-            threads = threads > 0 ? threads : 1;
-            cores = maxcpus / (sockets * dies * threads);
-        } else if (threads == 0) {
+        if (mc->smp_prefer_sockets) {
+            /* prefer sockets over cores before 6.2 */
+            if (sockets == 0) {
+                cores = cores > 0 ? cores : 1;
+                threads = threads > 0 ? threads : 1;
+                sockets = maxcpus / (dies * cores * threads);
+            } else if (cores == 0) {
+                threads = threads > 0 ? threads : 1;
+                cores = maxcpus / (sockets * dies * threads);
+            }
+        } else {
+            /* prefer cores over sockets since 6.2 */
+            if (cores == 0) {
+                sockets = sockets > 0 ? sockets : 1;
+                threads = threads > 0 ? threads : 1;
+                cores = maxcpus / (sockets * dies * threads);
+            } else if (sockets == 0) {
+                threads = threads > 0 ? threads : 1;
+                sockets = maxcpus / (dies * cores * threads);
+            }
+        }
+
+        /* try to calculate omitted threads at last */
+        if (threads == 0) {
             threads = maxcpus / (sockets * dies * cores);
         }
     }
diff --git a/hw/i386/pc_piix.c b/hw/i386/pc_piix.c
index 6cc834a..932e6e2 100644
--- a/hw/i386/pc_piix.c
+++ b/hw/i386/pc_piix.c
@@ -432,6 +432,7 @@ static void pc_i440fx_6_1_machine_options(MachineClass *m)
     m->is_default = false;
     compat_props_add(m->compat_props, hw_compat_6_1, hw_compat_6_1_len);
     compat_props_add(m->compat_props, pc_compat_6_1, pc_compat_6_1_len);
+    m->smp_prefer_sockets = true;
 }
 
 DEFINE_I440FX_MACHINE(v6_1, "pc-i440fx-6.1", NULL,
diff --git a/hw/i386/pc_q35.c b/hw/i386/pc_q35.c
index 5481d5c..57ab07e 100644
--- a/hw/i386/pc_q35.c
+++ b/hw/i386/pc_q35.c
@@ -372,6 +372,7 @@ static void pc_q35_6_1_machine_options(MachineClass *m)
     m->alias = NULL;
     compat_props_add(m->compat_props, hw_compat_6_1, hw_compat_6_1_len);
     compat_props_add(m->compat_props, pc_compat_6_1, pc_compat_6_1_len);
+    m->smp_prefer_sockets = true;
 }
 
 DEFINE_Q35_MACHINE(v6_1, "pc-q35-6.1", NULL,
diff --git a/hw/ppc/spapr.c b/hw/ppc/spapr.c
index b7bee5f..2986108 100644
--- a/hw/ppc/spapr.c
+++ b/hw/ppc/spapr.c
@@ -4685,6 +4685,7 @@ static void spapr_machine_6_1_class_options(MachineClass *mc)
     spapr_machine_6_2_class_options(mc);
     compat_props_add(mc->compat_props, hw_compat_6_1, hw_compat_6_1_len);
     smc->pre_6_2_numa_affinity = true;
+    mc->smp_prefer_sockets = true;
 }
 
 DEFINE_SPAPR_MACHINE(6_1, "6.1", false);
diff --git a/hw/s390x/s390-virtio-ccw.c b/hw/s390x/s390-virtio-ccw.c
index 61aeccb..5401c98 100644
--- a/hw/s390x/s390-virtio-ccw.c
+++ b/hw/s390x/s390-virtio-ccw.c
@@ -814,6 +814,7 @@ static void ccw_machine_6_1_class_options(MachineClass *mc)
 {
     ccw_machine_6_2_class_options(mc);
     compat_props_add(mc->compat_props, hw_compat_6_1, hw_compat_6_1_len);
+    mc->smp_prefer_sockets = true;
 }
 DEFINE_CCW_MACHINE(6_1, "6.1", false);