I'm settings these flags:
+3dnowprefetch,+abm,+adx,+aes,+arat,+avx,+avx2,+bmi1,+bmi2,+f16c,+fma,+lahf_lm,+movbe,+pclmulqdq,+popcnt,+rdrand,+rdseed,+rdtscp,+sep,+smap,+smep,+sse4.1,+sse4.2,+ssse3,+xsave,+xsaveopt,+kvm_pv_eoi
Which is way more than the Proxmox VE API allowed me to set as a flag...