Is get-slot-value guaranteed to be setf'able?

If I have a structure, my natural inclination is to set fields in the
structure using (setf (get-slot-value obj type field)).  In CMUCL,
this certainly works, because get-slot-value simply expands into a
call to alien:slot, which is setf'able.  My question is whether this
is considered "portable" UFFI, and will work on all implementations
which UFFI supports?  If this is not portable, is there some other way
I should be doing this?

This should maybe be mentioned briefly on p. 15 of the manual?

Cheers,

rif