Re: Is xl vcpu-set broken

[Anthony PERARD <anthony.perard@xxxxxxxxxx>]

On Tue, Feb 28, 2023 at 10:37:00AM +0100, Jan Beulich wrote:
> On 28.02.2023 07:44, Joe Jin wrote:
> > 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:
[...]
> > 
> > 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?
> 
> Judging from the DSDT we provide, offlining looks to also be intended to
> go the ACPI way. Whereas libxl only ever sends "device_add" commands to
> qemu, afaics (or "cpu-add" for older qemu). Anthony - do you have any
> insight what the intentions here are?

The intention is to one day implement cpu offline in QEMU upstream for
HVM guest, I don't think that's ever been done so far.

As we use device_add for cpu hotplug, we would probably do device_del
for hot-unplug, so qemu would still have to send the appropriate even to
the guest.

Someone will have to figure out if "device_del" works with a Xen guest,
doc here:
    https://www.qemu.org/docs/master/system/cpu-hotplug.html#vcpu-hot-unplug

Cheers,

-- 
Anthony PERARD