Re: [Qemu-ppc] [PATCH v3] KVM: remove buggy vcpu id check on vcpu creation

[Radim Krčmář]

2016-04-22 09:40+0800, Wanpeng Li:
> 2016-04-21 23:29 GMT+08:00 Radim Krčmář <address@hidden>:
>> x86 vcpu_id encodes APIC ID and APIC ID encodes CPU topology by
>> reserving blocks of bits for socket/core/thread, so if core or thread
>> count isn't a power of two, then the set of valid APIC IDs is sparse,
> 
>              ^^^^^^^^^^^^^^^^^^^
>              ^^^^^^^
> Is this the root reason why recommand max vCPUs per vm is 160 and the
> KVM_MAX_VCPUS is 255 instead of due to perforamnce concern?

No, the recommended amout of VCPUs is 160 because I didn't bump it after
PLE stopped killing big guests. :/

You can get full 255 VCPU guest with a proper configuration, e.g.
"-smp 255" or "-smp 255,cores=8" and the only problem is scalability,
but I don't know of anything that doesn't scale to that point.

(Scaling up to 2^32 is harder, because you don't want O(N) search, nor
 full allocation on smaller guests.  Neither is a big problem now.)