[libvirt] [PATCH] qemu: Fix retrieval of maximum number of vCPUs on KVM hosts

[Peter Krempa]

The detection of the maximum number of cpus used incorrect ioctl
argument value. This flaw caused that on kvm hosts this returns always
"160" as the maximum. This is just a recommended maximum value. The real
value is higher than that.

This patch tweaks the detection function to behave as described by the
kernel docs:

https://git.kernel.org/cgit/linux/kernel/git/torvalds/linux.git/tree/Documentation/virtual/kvm/api.txt?id=refs/tags/v3.9-rc2#n199
---
 src/qemu/qemu_driver.c | 29 +++++++++++++++++++----------
 1 file changed, 19 insertions(+), 10 deletions(-)

diff --git a/src/qemu/qemu_driver.c b/src/qemu/qemu_driver.c
index f4bbd74..7ca3e4c 100644
--- a/src/qemu/qemu_driver.c
+++ b/src/qemu/qemu_driver.c
@@ -1103,23 +1103,32 @@ static int qemuIsAlive(virConnectPtr conn ATTRIBUTE_UNUSED)
 }


-static int kvmGetMaxVCPUs(void) {
-    int maxvcpus = 1;
-
-    int r, fd;
+static int
+kvmGetMaxVCPUs(void) {
+    int fd;
+    int ret;

-    fd = open(KVM_DEVICE, O_RDONLY);
-    if (fd < 0) {
+    if ((fd = open(KVM_DEVICE, O_RDONLY)) < 0) {
         virReportSystemError(errno, _("Unable to open %s"), KVM_DEVICE);
         return -1;
     }

-    r = ioctl(fd, KVM_CHECK_EXTENSION, KVM_CAP_NR_VCPUS);
-    if (r > 0)
-        maxvcpus = r;
+    /* at first try KVM_CAP_MAX_VCPUS to determine the maximum count */
+    if ((ret = ioctl(fd, KVM_CHECK_EXTENSION, KVM_CAP_MAX_VCPUS)) > 0)
+        goto cleanup;
+
+    /* as a fallback get KVM_CAP_NR_VCPUS (the recommended maximum number of
+     * vcpus). Note that on most machines this is set to 160. */
+    if ((ret = ioctl(fd, KVM_CHECK_EXTENSION, KVM_CAP_NR_VCPUS)) > 0)
+        goto cleanup;
+
+    /* if KVM_CAP_NR_VCPUS doesn't exist either, kernel documentation states
+     * that 4 should be used as the maximum number of cpus */
+    ret = 4;

+cleanup:
     VIR_FORCE_CLOSE(fd);
-    return maxvcpus;
+    return ret;
 }


-- 
1.8.1.5