Quoting Daniel P. Berrange (berrange@redhat.com):

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

commit b07f3d821dfb11a118ee75ea275fd6ab737d9500 Author: Daniel P. Berrange <berrange@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@redhat.com>

Thanks, confirmed this let's me place qemu in a cpuset. (Noting that 'virsh edit' always ends up squashing the cputune section when the cpusets are identical, but that should all be reasonable) Acked-by: Serge E. Hallyn <serge.hallyn@ubuntu.com> Tested-by: Serge E. Hallyn <serge.hallyn@ubuntu.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