xapi_vm_helpers: Raise allowed_VIF limit from 7 to 16

With the previously hardcoded value of max_grant_frames, we could only support
7 VIFs at most (and fewer if there were also many VBDs), since having 9 and
more VIFs would result in grant allocation errors.

Now that max_grant_frames is dynamically estimated on domain creation given the
number of VIFs and VBDs a VM has, we can easily support 16 VIFs.

Given the current behaviour of the XenServer/XCP-ng system
(hypervisor+drivers), where more VIFs allow for higher overall networking
throughput, this is highly beneficial - in testing overall throughput with 16
VIFs was 18-27% higher than with 8 VIFs (tested with multiple iperf3 instances
running on all interfaces simultaneously)

Moreover, some users coming from VMWare are used to networking setups with
dozens of VIFs, and this is a step towards allowing that without encountering
any other bottlenecks in the system.

NOTE: We are currently only allocating enough grants for 1 hotplugged VIF
above 7. Therefore, technically we shouldn't allow creating more than one VIF
when the VM is running (or paused), but there is currently no way in xapi to
check how many and which VIFs were hotplugged as far as I know, and
allowed_VIFs are honoured by clients on VIF creation, not hotplug.
Since this is in keeping with the previous behaviour of this field (if VM has
many VBDs, it wouldn't have been able to allocate 7 VIFs before) as an
"advice", and not a "guarantee" or "limit", I've decided to keep it as-is.
A more detailed technical explanation of the supported limit should be
described elsewhere in support statements.

Signed-off-by: Andrii Sultanov <[email protected]>

Author: last-genius

ocaml/xapi/xapi_vm_helpers.ml

@@ -1304,9 +1304,9 @@ let allowed_VBD_devices_HVM_floppy =
     (fun x -> Device_number.(make Floppy ~disk:x ~partition:0))
     (inclusive_range 0 1)
 
-let allowed_VIF_devices_HVM = vif_inclusive_range 0 6
+let allowed_VIF_devices_HVM = vif_inclusive_range 0 15
 
-let allowed_VIF_devices_PV = vif_inclusive_range 0 6
+let allowed_VIF_devices_PV = vif_inclusive_range 0 15
 
 (** [possible_VBD_devices_of_string s] returns a list of Device_number.t which
     	represent possible interpretations of [s]. *)