For our administration, we need the following actions, while concerned domain is running: 1) Change the number of virtual CPUs. 2) Change the pinning (affinity) of a virtual CPU on real CPUs. 3) Get detailed information for each virtual CPU. Currently there is no Libvirt function provided for that. We suggest to add the following 3 functions (in libvirt.c): /** * virDomainSetVcpus: * @domain: pointer to domain object, or NULL for Domain0 * @nvcpus: the new number of virtual CPUs for this domain * * Dynamically change the number of virtual CPUs used by the domain. * Note that this call may fail if the underlying virtualization hypervisor * does not support it or if growing the number is arbitrary limited. * This function requires priviledged access to the hypervisor. * * Returns 0 in case of success, -1 in case of failure. */ int virDomainSetVcpus(virDomainPtr domain, unsigned int nvcpus) /** * virDomainPinVcpu: * @domain: pointer to domain object, or NULL for Domain0 * @vcpu: virtual CPU number * @cpumap: pointer to a bit map of real CPUs (format in virVcpuInfo) * @maplen: length of cpumap, in 8-bit bytes * * Dynamically change the real CPUs which can be allocated to a virtual CPU. * This function requires priviledged access to the hypervisor. * * Returns 0 in case of success, -1 in case of failure. */ int virDomainPinVcpu(virDomainPtr domain, unsigned int vcpu, unsigned char *cpumap, int maplen) /** * virDomainGetVcpus: * @domain: pointer to domain object, or NULL for Domain0 * @info: pointer to an array of virVcpuInfo structures * @maxinfo: number of structures in info array * * Extract information about virtual CPUs of a domain, store it in info array. * * Returns the number of info filled in case of success, -1 in case of failure. */ int virDomainGetVcpus(virDomainPtr domain, virVcpuInfoPtr info, int maxinfo) with the following structure (in libvirt.h): /** * virVcpuInfo: structure for information about a virtual CPU in a domain. */ #define VIR_MAX_CPUS 256 typedef enum { VIR_VCPU_OFFLINE = 0, /* the virtual CPU is offline */ VIR_VCPU_RUNNING = 1, /* the virtual CPU is running */ VIR_VCPU_BLOCKED = 2, /* the virtual CPU is blocked on resource */ } virVcpuState; typedef struct _virVcpuInfo virVcpuInfo; struct _virVcpuInfo { unsigned int number; /* virtual CPU number */ int state; /* value from virVcpuState */ unsigned long long cpuTime; /* CPU time used, in nanoseconds */ int cpu; /* real CPU number, or -1 if offline */ unsigned char cpumap[VIR_MAX_CPUS/8]; /* Bit map of usable real CPUs. Each bit set to 1 means that corresponding CPU is usable. Bytes are stored in little-endian order: CPU0-7, 8-15... In each byte, lowest CPU number is least significant bit */ }; typedef virVcpuInfo *virVcpuInfoPtr; I have successfully tried those functions via Xen hypervisor, except for the first (SetVcpus) where hypervisor operation DOM0_MAX_VCPUS fails (maybe it is not possible on a running domain ?). That function was successful via Xen daemon.