Is xl vcpu-set broken

[Joe Jin <joe.jin@xxxxxxxxxx>]

Hi,

We encountered a vcpu-set issue on old xen, when I tried to confirm
if xen upstream xen has the same issue I find neither my upstream build
nor ubuntu 22.04 xen-hypervisor-4.16 work.

I can add vcpus(8->16) to my guest but I can not reduce vcpu number:

root@ubuntu2204:~/vm# xl list
Name                                        ID   Mem VCPUs    State    Time(s)
Domain-0                                     0 255424    32     r-----     991.9
testvm                                       1   4088    16     -b----      94.6
root@ubuntu2204:~/vm# xl vcpu-set testvm 8
root@ubuntu2204:~/vm# xl list
Name                                        ID   Mem VCPUs    State    Time(s)
Domain-0                                     0 255424    32     r-----     998.5
testvm                                       1   4088    16     -b----      97.3

After xl vcpu-set, xenstore showed online cpu number reduced to 8:

/local/domain/1/vm = "/vm/aa109ea0-2fde-4712-81d8-75f73bd73715"
/local/domain/1/name = "testvm"
/local/domain/1/cpu = ""
/local/domain/1/cpu/0 = ""
/local/domain/1/cpu/0/availability = "online"
/local/domain/1/cpu/1 = ""
/local/domain/1/cpu/1/availability = "online"
/local/domain/1/cpu/2 = ""
/local/domain/1/cpu/2/availability = "online"
/local/domain/1/cpu/3 = ""
/local/domain/1/cpu/3/availability = "online"
/local/domain/1/cpu/4 = ""
/local/domain/1/cpu/4/availability = "online"
/local/domain/1/cpu/5 = ""
/local/domain/1/cpu/5/availability = "online"
/local/domain/1/cpu/6 = ""
/local/domain/1/cpu/6/availability = "online"
/local/domain/1/cpu/7 = ""
/local/domain/1/cpu/7/availability = "online"
/local/domain/1/cpu/8 = ""
/local/domain/1/cpu/8/availability = "offline"
/local/domain/1/cpu/9 = ""
/local/domain/1/cpu/9/availability = "offline"
/local/domain/1/cpu/10 = ""
/local/domain/1/cpu/10/availability = "offline"
/local/domain/1/cpu/11 = ""
/local/domain/1/cpu/11/availability = "offline"
/local/domain/1/cpu/12 = ""
/local/domain/1/cpu/12/availability = "offline"
/local/domain/1/cpu/13 = ""
/local/domain/1/cpu/13/availability = "offline"
/local/domain/1/cpu/14 = ""
/local/domain/1/cpu/14/availability = "offline"
/local/domain/1/cpu/15 = ""
/local/domain/1/cpu/15/availability = "offline"
/local/domain/1/cpu/16 = ""
/local/domain/1/cpu/16/availability = "offline"

But guest did not received any offline events at all.

>From source code my understand is for cpu online, libxl will
send device_add to qemu to trigger vcpu add, for cpu offline,
it still rely on xenstore, is this correct? anything can block
cpu scale down?

Appreciate for any suggestions!

Thanks,
Joe