[Qemu-devel] [PATCH v2] kvm: warn if num cpus is greater than num recommended

[Andrew Jones <drjones@redhat.com>]

The comment in kvm_max_vcpus() states that it's using the recommended
procedure from the kernel API documentation to get the max number
of vcpus that kvm supports. It is, but by always returning the
maximum number supported. The maximum number should only be used
for development purposes. qemu should check KVM_CAP_NR_VCPUS for
the recommended number of vcpus. This patch adds a warning if a user
specifies a number of cpus between the recommended and max.

v2:
Incorporate tests for max_cpus, which specifies the maximum number
of hotpluggable cpus. An additional note is that the message for
the fail case was slightly changed, 'exceeds max cpus' to
'exceeds the maximum cpus'. If this is unacceptable change for
users like libvirt, then I'll need to spin a v3.

Signed-off-by: Andrew Jones <drjones@redhat.com>
---
 kvm-all.c | 69 ++++++++++++++++++++++++++++++++++++---------------------------
 1 file changed, 40 insertions(+), 29 deletions(-)

diff --git a/kvm-all.c b/kvm-all.c
index a2d49786365e3..021f5f47e53da 100644
--- a/kvm-all.c
+++ b/kvm-all.c
@@ -1322,24 +1322,20 @@ static int kvm_irqchip_create(KVMState *s)
     return 0;
 }
 
-static int kvm_max_vcpus(KVMState *s)
+/* Find number of supported CPUs using the recommended
+ * procedure from the kernel API documentation to cope with
+ * older kernels that may be missing capabilities.
+ */
+static int kvm_recommended_vcpus(KVMState *s)
 {
-    int ret;
-
-    /* Find number of supported CPUs using the recommended
-     * procedure from the kernel API documentation to cope with
-     * older kernels that may be missing capabilities.
-     */
-    ret = kvm_check_extension(s, KVM_CAP_MAX_VCPUS);
-    if (ret) {
-        return ret;
-    }
-    ret = kvm_check_extension(s, KVM_CAP_NR_VCPUS);
-    if (ret) {
-        return ret;
-    }
+    int ret = kvm_check_extension(s, KVM_CAP_NR_VCPUS);
+    return (ret) ? ret : 4;
+}
 
-    return 4;
+static int kvm_max_vcpus(KVMState *s)
+{
+    int ret = kvm_check_extension(s, KVM_CAP_MAX_VCPUS);
+    return (ret) ? ret : kvm_recommended_vcpus(s);
 }
 
 int kvm_init(void)
@@ -1347,11 +1343,19 @@ int kvm_init(void)
     static const char upgrade_note[] =
         "Please upgrade to at least kernel 2.6.29 or recent kvm-kmod\n"
         "(see http://sourceforge.net/projects/kvm).\n";
+    struct {
+        const char *name;
+        int num;
+    } num_cpus[] = {
+        { "SMP",          smp_cpus },
+        { "hotpluggable", max_cpus },
+        { NULL, }
+    }, *nc = num_cpus;
+    int soft_vcpus_limit, hard_vcpus_limit;
     KVMState *s;
     const KVMCapabilityInfo *missing_cap;
     int ret;
     int i;
-    int max_vcpus;
 
     s = g_malloc0(sizeof(KVMState));
 
@@ -1392,19 +1396,26 @@ int kvm_init(void)
         goto err;
     }
 
-    max_vcpus = kvm_max_vcpus(s);
-    if (smp_cpus > max_vcpus) {
-        ret = -EINVAL;
-        fprintf(stderr, "Number of SMP cpus requested (%d) exceeds max cpus "
-                "supported by KVM (%d)\n", smp_cpus, max_vcpus);
-        goto err;
-    }
+    /* check the vcpu limits */
+    soft_vcpus_limit = kvm_recommended_vcpus(s);
+    hard_vcpus_limit = kvm_max_vcpus(s);
 
-    if (max_cpus > max_vcpus) {
-        ret = -EINVAL;
-        fprintf(stderr, "Number of hotpluggable cpus requested (%d) exceeds max cpus "
-                "supported by KVM (%d)\n", max_cpus, max_vcpus);
-        goto err;
+    while (nc->name) {
+        if (nc->num > soft_vcpus_limit) {
+            fprintf(stderr,
+                    "Warning: Number of %s cpus requested (%d) exceeds "
+                    "the recommended cpus supported by KVM (%d)\n",
+                    nc->name, nc->num, soft_vcpus_limit);
+
+            if (nc->num > hard_vcpus_limit) {
+                ret = -EINVAL;
+                fprintf(stderr, "Number of %s cpus requested (%d) exceeds "
+                        "the maximum cpus supported by KVM (%d)\n",
+                        nc->name, nc->num, hard_vcpus_limit);
+                goto err;
+            }
+        }
+        nc++;
     }
 
     s->vmfd = kvm_ioctl(s, KVM_CREATE_VM, 0);
-- 
1.8.1.4