[libvirt] [RFC] New API to retrieve node CPU map

[Viktor Mihajlovski]

Hi,

in order to use the APIs listed below it is necessary for a client to 
know the maximum number of node CPUs which is passed via the maplen 
argument.

virDomainGetEmulatorPinInfo
virDomainGetVcpuPinInfo
virDomainGetVcpus
virDomainPinEmulator
virDomainPinVcpu
virDomainPinVcpuFlags

The current approach uses virNodeGetInfo to determine the maximum CPU 
number. This can lead to incorrect results if not all node CPUs are 
online. The maximum CPU number should always be the number of CPUs 
present on the host, regardless of their online/offline state.

The following example illustrates the issue:
Host has 3 logical CPUs, 1 socket, 3 cores, 1 thread.
Guest has 1 virtual CPU and is started while all 3 host CPUs are
online.

$ virsh vcpuinfo guest
VCPU:           0
CPU:            0
State:          running
CPU time:       35.4s
CPU Affinity:   yyy

$ echo 0 > /sys/devices/system/cpu/cpu1/online

$ virsh vcpuinfo guest
VCPU:           0
CPU:            0
State:          running
CPU time:       35.5s
CPU Affinity:   y-

The correct display for CPU affinity would have been y-y, as the guest
continues to use CPUs 0 and 2.
This is not a display problem only, because it is also not possible to
explicitly pin the virtual CPU to host CPUs 0 and 2, due to the
truncated CPU mask.

PROPOSAL:

To help solve the issue above I suggest two new public API functions:

int
virNodeGetCpuNum(virConnectPtr conn);

returning the number of present CPUs on the host or -1 upon failure
and

int
virNodeGetCpuMap(virConnectPtr conn, unsigned char * cpumap, int maplen);

returning the number of present CPUs or -1 on failure and storing a bit 
map of real CPUs as described in virDomainPinVcpu in cpumap. The bits in 
the bit map are set to 1 for online CPUs and set to 0 for offline CPUs.

Implementation is facilitated by the function nodeGetCPUmap in nodeinfo.c.

Clients can use virNodeGetCpuNum to properly determine the maximum 
number of node CPUs and the online/offline information.

Thanks for your comments.

-- 

Mit freundlichen Grüßen/Kind Regards
    Viktor Mihajlovski

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