Re: Faking the number of CPUs for dom0 with dom0_max_vcpus

[Jan Beulich <jbeulich@xxxxxxxx>]

On 12.11.2023 21:28, Jimmy Lee wrote:
> Hello Xen experts, I am trying to set dom0_max_vcpus to a number that is
> larger than the number of pcpus. For example, I have a 4-CPU machine but I
> want applications in dom0 to have an illusion that they are running on a
> 64-cpu machine. However, it seems that dom0 will always recognize the
> number of pcpus. I can see the vcpus in the output of "xl vcpu-list" but
> most of them are not online. How can I achieve my goal? Thanks!

Even if I was to assume the Dom0 kernel is Linux, it would still remain
unclear what Linux version you use. Generally with recent Linux kernels
this has been working for me (albeit I didn't try in a while, after
putting together what has become e25a8d959992 ["x86/Xen: streamline (and
fix) PV CPU enumeration"]). If despite having this in your Linux you
still don't see this working, you will want to supply applicable logs.

As an aside - 64 vCPU-s on a 4-pCPU system looks excessive to me.

Jan

> [root@ip-10-0-131-61 cpu3]# xl vcpu-list
>> Name                                ID  VCPU   CPU State   Time(s)
>> Affinity (Hard / Soft)
>> Domain-0                             0     0    0   r--      15.9  all /
>> all
>> Domain-0                             0     1    3   -b-      11.9  all /
>> all
>> Domain-0                             0     2    1   -b-      11.2  all /
>> all
>> Domain-0                             0     3    2   -b-      12.4  all /
>> all
>> Domain-0                             0     4    -   --p       0.0  all /
>> all
>> Domain-0                             0     5    -   --p       0.0  all /
>> all
>> Domain-0                             0     6    -   --p       0.0  all /
>> all
>> Domain-0                             0     7    -   --p       0.0  all /
>> all
>> Domain-0                             0     8    -   --p       0.0  all /
>> all
>> ....
>>
> 
> [root@ip-10-0-131-61 cpu3]# lscpu
> Architecture:          x86_64
> CPU op-mode(s):        32-bit, 64-bit
> Byte Order:            Little Endian
> CPU(s):                4
> On-line CPU(s) list:   0-3
> Thread(s) per core:    4
> Core(s) per socket:    1
> Socket(s):             1
> NUMA node(s):          1
> ...
> 
> [root@ip-10-0-131-61 cpu3]# xl info
> ...
> xen_commandline        : placeholder dom0_mem=3%,min:2G,max:4G
> com1=115200,8n1 console=com1,tty,vga dom0_max_vcpus=64 spec-ctrl=no
> sched=credit vcpu_migration_delay=1000 allowsuperpage ioapic_ack=new
> ...
>