I think that rather than rubber-stamping "let's send things and see what
happens", it makes more sense to document that there are current
implementations which do work that way, but that we don't recommend
that. As in:
(...); however, note that implementations exist which support the
command even without advertising this bit (...)
In the long term, I would prefer that we don't add new implementations
which behave in this manner, while not immediately causing the existing
implementation (i.e., xnbd) to suddenly no longer be compliant. This
wording does do that.
[...]
+* `NBD_CMD_CACHE` (5)
[...]
+ A client MAY attempt to send a cache request even when
+ `NBD_FLAG_SEND_CACHE` was not set in the transmission flags field,
"Note that client implementations exist which attempt to send a cache
request even when (...)"
+ however, in that case, it MUST NOT use any command flags. A
"but these implementations do not use any command flags."
other than that, looks good to me.