[Xen-API] RE: VM. get allowed VBD devices

[Dave Scott <Dave.Scott@xxxxxxxxxxxxx>]

Hi,

> What exactly is VM. get allowed VBD devices()? What list does it return?

The return value is a list of possible "VBD.userdevice" values: integers from 0 
up to some supported maximum (where the maximum depends on guest type), with 
entries corresponding to existing VBDs removed.

The purpose is to give clients a hint about what "VBD.userdevice" values are 
possible for a given VM configuration.

The supported maximums are hardcoded in ocaml/xapi/xapi_vm_helpers.ml:

let allowed_VBD_devices_HVM    = range_inclusive 0 3
let allowed_VBD_devices_HVM_PP = range_inclusive 0 15
let allowed_VBD_devices_PV     = range_inclusive 0 15
let allowed_VIF_devices_HVM    = range_inclusive 0 3
let allowed_VIF_devices_HVM_PP = range_inclusive 0 6
let allowed_VIF_devices_PV     = range_inclusive 0 6

where "PV" means fully paravirt linux guests; "HVM" means HVM guests with no PV 
drivers; "HVM_PP" means HVM guests with PV drivers.

Note that none of these limits are actually enforced; this is just a hinting 
mechanism.

Cheers,
Dave

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