Kernel log "xen:balloon: Cannot add additional memory (-22)" every 3670 seconds

[Andy Smith <andy@xxxxxxxxxxxxxx>]

Hi,

I have several machines on Xen 4.20.2 / Debian 13 that were upgraded
from Xen 4.14.5. One of these machines does the following pair of kernel
logs every 3670 seconds:

[44081.525564] Block size [0x80000000] unaligned hotplug range: start 
0x1600000000, size 0x8000000
[44081.525574] xen:balloon: Cannot add additional memory (-22)

It is always this pair of lines exactly every 3670 seconds. The values
"0x80000000" and "0x1600000000" are the same every time.

There is only one machine doing this, but it has slightly different
hardware from the others so I do not know if it's down to that. It did
not do it when it was running Xen 4.14.5 though. It happens whether the
machine is running any additional guests or not (I realise that dom0 is
a guest).

The thing is, ballooning is disabled and dom0 has a fixed amount of RAM:

$ grep autoballoon /etc/xen/xl.conf
autoballoon="off"

$ sudo xl info | grep xen_commandline
xen_commandline        : placeholder dom0_mem=8192M dom0_max_vcpus=2 
com1=115200,8n1,0x2f8,10 console=com1,vga ucode=scan smt=1 no-real-mode edd=off

(I didn't specify "no-real-mode edd=off"; it seems those were added by
something else and are present on all other machines too.)

The dom0 kernel's command line is unremarkable:

$ cat /proc/cmdline
placeholder root=UUID=1686c413-d3df-4b17-ad39-957991e0fbeb ro console=tty0 
consoleblank=0 console=hvc0

The Xen software on this machine is self-compiled, not Debian's
packages.

Does anyone have any insight into what happens every 3670 seconds
exactly to cause this kernel log?

Is it a cause for concern? I haven't noticed any issues.

Searching around has only led me to very old threads (one of which I was
in!) about similar but not identical logs, so I don't think any of those
are related.

Thanks,
Andy