I am replying since Olivier liked to hear from others with a lot of cores.
We are running XCP-ng 8.1 on a host with dual AMD Epyc 7713 64 core processors. With hyperthreading it is a total of 256 hyperthreading cores. Since we only are able to assign up to 128 cores to a VM, we have turned hyperthreading off. The VM is running Ubuntu 18. We should probably lower the number of vcpus to 120 or something for best performance, but at the moment it is 128.
We can see in the Performance Graphs that all the cores are active:
The output of lscpu is:
lspci
00:00.0 Host bridge: Intel Corporation 440FX - 82441FX PMC [Natoma] (rev 02)
00:01.0 ISA bridge: Intel Corporation 82371SB PIIX3 ISA [Natoma/Triton II]
00:01.1 IDE interface: Intel Corporation 82371SB PIIX3 IDE [Natoma/Triton II]
00:01.2 USB controller: Intel Corporation 82371SB PIIX3 USB [Natoma/Triton II] ( rev 01)
00:01.3 Bridge: Intel Corporation 82371AB/EB/MB PIIX4 ACPI (rev 01)
00:02.0 VGA compatible controller: Device 1234:1111
00:03.0 SCSI storage controller: XenSource, Inc. Xen Platform Device (rev 02)
hansb@FVCOM-U18:~$ lscpu
Architecture: x86_64
CPU op-mode(s): 32-bit, 64-bit
Byte Order: Little Endian
CPU(s): 128
On-line CPU(s) list: 0-127
Thread(s) per core: 1
Core(s) per socket: 64
Socket(s): 2
NUMA node(s): 1
Vendor ID: AuthenticAMD
CPU family: 25
Model: 1
Model name: AMD EPYC 7713 64-Core Processor
Stepping: 1
CPU MHz: 1996.267
BogoMIPS: 3992.57
Hypervisor vendor: Xen
Virtualization type: full
L1d cache: 32K
L1i cache: 32K
L2 cache: 512K
L3 cache: 262144K
NUMA node0 CPU(s): 0-127
Flags: fpu vme de pse tsc msr pae mce cx8 apic sep mtrr pge mca cm ov pat pse36 clflush mmx fxsr sse sse2 ht syscall nx mmxext fxsr_opt pdpe1gb rdt scp lm rep_good nopl cpuid extd_apicid pni pclmulqdq ssse3 fma cx16 sse4_1 sse4_ 2 x2apic movbe popcnt aes xsave avx f16c rdrand hypervisor lahf_lm cmp_legacy cr 8_legacy abm sse4a misalignsse 3dnowprefetch bpext ibpb vmmcall fsgsbase bmi1 av x2 smep bmi2 rdseed adx smap clflushopt clwb sha_ni xsaveopt xsavec xgetbv1 xsav es clzero xsaveerptr arat umip rdpid