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+id = 2848
+title = "i386 max_cpus off by one"
+state = "opened"
+created_at = "2025-03-02T00:58:09.053Z"
+closed_at = "n/a"
+labels = ["target: i386"]
+url = "https://gitlab.com/qemu-project/qemu/-/issues/2848"
+host-os = "NixOS"
+host-arch = "x86"
+qemu-version = "9.1.2"
+guest-os = "n/a"
+guest-arch = "x86"
+description = """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."""
+reproduce = "n/a"
+additional = "n/a"