Actual dom0 memory much lower than dom0_mem setting

[Andy Smith <andy@xxxxxxxxxxxxxx>]

Hi,

Something I noticed when looking into my previous issue: Despite
dom0_memory being set to 8192M on the hypervisor command line, and
ballooning being disabled, with Xen 4.20.2 dom0 comes up with only 6353M
available to it (according to "free -m").

So, aiming to get a usable 8192M dom0 memory, I bumped it up by the
shortfall of 1839M:

$ sudo xl info | grep xen_commandline
xen_commandline        : placeholder dom0_mem=10031M dom0_max_vcpus=2 
com1=115200,8n1,0x2f8,10 console=com1,vga ucode=scan smt=1 no-real-mode edd=off
$ free -m
               total        used        free      shared  buff/cache   available
Mem:            7794         400        7286          12         228        7394
Swap:           1904           0        1904

I ended up with only 7794M. So it seems like the more memory I give
dom0, the bigger the shortfall gets.

It is predictable in that if I specify 10031M on this machine I always
end up with 7794M usable, but I can't predict what it will be ahead of
time so I don't know what to set it to in order to end up with 8192M.

This happens on other machines as well so I think it must be pretty
widespread.

This sort of thing has happened forever — well before Xen 4.14. Here's
an example from a different machine, slightly different hardware, Xen
4.14.5 with dom0 kernel 5.10.0-0.deb10.16-amd64 (very soon to be
reinstalled/upgraded!):

$ 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
$ free -m
              total        used        free      shared  buff/cache   available
Mem:           6285        1425        3239         126        1620        4642
Swap:           952           1         951

Is this widely know and expected?

Is it fixable?

Is it something you can calculate? ("To get 8192M usable dom0 memory I
must specify…").

Thanks,
Andy