[libvirt] [PATCH] qemu: Allow use of hot plugged host CPUs if no affinity set

[Viktor Mihajlovski]

Running VMs couldn't use newly hot plugged host CPUs even if the VMs
had no CPU pinning defined AND the cpuset controller was disabled in
the libvirt qemu configuration.
This was because in this case the process affinity was set by libvirt
to all currently present host CPUs in order to avoid situations, where
libvirtd was deliberately running on a CPU subset and thus the spawned
VMs would be involuntarily restricted to the CPU subset inherited by
libvirtd.
That however prevents new host CPUs to be utilized when they show up.
With this change we will NOT set the VM's affinity mask if it
matches the online host CPU mask.

There's still the chance that for some reason the deliberately chosen
libvirtd affinity matches the online host CPU mask by accident. In this
case the behavior remains as it was before (CPUs offline while setting
the affinity will not be used if they show up later on).

Signed-off-by: Viktor Mihajlovski <mihajlov at linux.vnet.ibm.com>
Tested-by: Matthew Rosato <mjrosato at linux.vnet.ibm.com>
---
 src/qemu/qemu_process.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index 1b67aee..76f9210 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -2202,6 +2202,7 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm)
     int ret = -1;
     virBitmapPtr cpumap = NULL;
     virBitmapPtr cpumapToSet = NULL;
+    virBitmapPtr hostcpumap = NULL;
     qemuDomainObjPrivatePtr priv = vm->privateData;
 
     if (!vm->pid) {
@@ -2223,6 +2224,16 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm)
              * the spawned QEMU instance to all pCPUs if no map is given in
              * its config file */
             int hostcpus;
+            hostcpumap = virHostCPUGetOnlineBitmap();
+            cpumap = virProcessGetAffinity(vm->pid);
+
+            if (hostcpumap && cpumap && virBitmapEqual(hostcpumap, cpumap)) {
+                /* we're using all available CPUs, no reason to set
+                 * mask. If libvirtd is running without explicit
+                 * affinity, we can use hotplugged CPUs for this VM */
+                ret = 0;
+                goto cleanup;
+            }
 
             /* setaffinity fails if you set bits for CPUs which
              * aren't present, so we have to limit ourselves */
@@ -2248,6 +2259,7 @@ qemuProcessInitCpuAffinity(virDomainObjPtr vm)
 
  cleanup:
     virBitmapFree(cpumap);
+    virBitmapFree(hostcpumap);
     return ret;
 }
 
-- 
1.9.1