i386: 0.981
performance: 0.965
x86: 0.955
VMM: 0.894
graphic: 0.758
device: 0.691
architecture: 0.665
semantic: 0.657
vnc: 0.558
kernel: 0.484
socket: 0.461
ppc: 0.438
risc-v: 0.330
virtual: 0.326
PID: 0.317
network: 0.305
boot: 0.303
mistranslation: 0.286
arm: 0.255
KVM: 0.214
register: 0.177
TCG: 0.177
hypervisor: 0.169
files: 0.168
permissions: 0.166
debug: 0.132
user-level: 0.119
assembly: 0.093
peripherals: 0.065

i386 max_cpus off by one
Description of problem:
X86 VMs are currently limited to 255 vCPUs (`mc->max_cpus = 255;` in `pc.c`).
The first occurrence i can find of this limit is in d3e9db933f416c9f1c04df4834d36e2315952e42 from 2005 where both `MAX_APICS` and `MAX_CPUS` was set to 255. This is becoming relevant for some people as servers with 256 cores become more available. 

**Can we increase the limit to 256 vCPUs?** 
I think so. 

Today, the APIC id limit (see `apic_id_limit` in `x86-common.c`) is based on the CPU id limit. 
According to the a comment for `typdef uint32_t apic_id_t;` (see `topology.h`), we can have 256 APICs, but more APICs require x2APIC support. 
APIC seems to be no hindrance to increase max_cpus to 256. 

**Can we increase the limit to 512?** Maybe not? We need x2APIC support of which i have no clue. Also there is always a performance risk of exceeding the size at which current data structures work efficiently.