Re: [virtio-comment] Re: [PATCH v3 4/4] Add CCW configuration field "indirect_num"

[Cornelia Huck <cohuck@redhat.com>]

On Sat, Mar 19 2022, Christian Schoenebeck <qemu_oss@crudebyte.com> wrote:

> On Freitag, 18. MÃrz 2022 17:06:25 CET Halil Pasic wrote:

>> I agree that the "including" is important, but I'm not sure about the
>> "its contents are undefined". I don't really understand why should we use
>> a plural here. What speaks against specifying that in SHOULD be stored
>> as 0 by the device, and MUST be ignored by the driver?
>
> Both solutions would be viable. Personally I would just use something like 
> "Should be zero" if there is a value in recommending that, but I don't see a 
> value in recommending to set something to zero and at the same time requiring 
> to not access it in the first place.
>
>> Currently we say that \field{max_indirect_num} exists like a be32 field
>> even if VIRTIO_RING_F_INDIRECT_SIZE is not negotiated. Which kind of
>> implies that at least type invariants should hold. Of course, there is
>> none here (i.e. every bits value is also a be32 value), but for something
>> like an enum interesting corner cases can pop up.
>
> I can't follow you on that one. What has that do with enums in this case?
>
> Anyway, I won't persist on my suggestion to use the (IMO more compact form) 
> "undefined". If you guys prefer the more specific solution "SHOULD be 0 and 
> MUST not be accessed" then I will go that way.

I'm not sure what mandating 0 and non-access would buy us here... the
driver can of course read the field (e.g. when copying the structure
wholesale); it just can't make use of the contents when it did not
negotiate the feature (but why would it do so in that case anyway?)

Also, I think junk remains junk, whether it is a be32 field or
interpreted as an enum. It is simply not valid, even if it might by
accident end up to be a defined enum value.

So I think "undefined" should be fine.