[libvirt] [PATCH] qemu: fix setting of VM CPU affinity with TCG

[Daniel P. Berrange]

If a previous commit I fixed the incorrect handling of vcpu pids
for TCG mode QEMU:

  commit b07f3d821dfb11a118ee75ea275fd6ab737d9500
  Author: Daniel P. Berrange <berrange at redhat.com>
  Date:   Thu Dec 18 16:34:39 2014 +0000

    Don't setup fake CPU pids for old QEMU

    The code assumes that def->vcpus == nvcpupids, so when we setup
    fake CPU pids for old QEMU with nvcpupids == 1, we cause the
    later code to read off the end of the array. This has fun results
    like sche_setaffinity(0, ...) which changes libvirtd's own CPU
    affinity, or even better sched_setaffinity($RANDOM, ...) which
    changes the affinity of a random OS process.

The intent was that this would merely disable the ability to set
per-vCPU affinity. It should still have been possible to set VM
level host CPU affinity.

Unfortunately, when you set  <vcpu cpuset='0-1'>4</vcpu>, the XML
parser will internally take this & initialize an entry in the
def->cputune.vcpupin array for every VCPU. IOW this is implicitly
being treated as

  <cputune>
    <vcpupin cpuset='0-1' vcpu='0'/>
    <vcpupin cpuset='0-1' vcpu='1'/>
    <vcpupin cpuset='0-1' vcpu='2'/>
    <vcpupin cpuset='0-1' vcpu='3'/>
  </cputune>

Even more fun, the faked cputune elements are hidden from view when
querying the live XML, because their cpuset mask is the same as the
VM default cpumask.

The upshot was that it was impossible to set VM level CPU affinity.

To fix this we must update qemuProcessSetVcpuAffinities so that it
only reports a fatal error if the per-VCPU cpu mask is different
from the VM level cpu mask.

Signed-off-by: Daniel P. Berrange <berrange at redhat.com>
---
 src/qemu/qemu_process.c | 15 ++++++++++++---
 1 file changed, 12 insertions(+), 3 deletions(-)

diff --git a/src/qemu/qemu_process.c b/src/qemu/qemu_process.c
index d5df60d..e8c532f 100644
--- a/src/qemu/qemu_process.c
+++ b/src/qemu/qemu_process.c
@@ -2497,9 +2497,18 @@ qemuProcessSetVcpuAffinities(virDomainObjPtr vm)
         return 0;
 
     if (priv->vcpupids == NULL) {
-        virReportError(VIR_ERR_OPERATION_INVALID,
-                       "%s", _("cpu affinity is not supported"));
-        return -1;
+        /* If any CPU has custom affinity that differs from the
+         * VM default affinity, we must reject it
+         */
+        for (n = 0; n < def->vcpus; n++) {
+            if (!virBitmapEqual(def->cpumask,
+                                def->cputune.vcpupin[n]->cpumask)) {
+                virReportError(VIR_ERR_OPERATION_INVALID,
+                               "%s", _("cpu affinity is not supported"));
+                return -1;
+            }
+        }
+        return 0;
     }
 
     for (n = 0; n < def->vcpus; n++) {
-- 
2.1.0