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Subject: Re: [virtio] Re: [PATCH v9 14/16] VIRTIO_F_NOTIFICATION_DATA: extra data to devices

On 03/07/2018 03:49 PM, Cornelia Huck wrote:
>>>> +When VIRTIO_F_NOTIFICATION_DATA has been negotiated,
>>>> +the driver notifies the device by writing the following
>>>> +32-bit value to the Queue Notify address:
>>>> +\begin{lstlisting}
>>>> +le32 vqn : 16,
>>>> +     next_off : 15,
>>>> +     next_wrap : 1;  
>>> Don't we want to write this as
>>> le32 vqn : 16;
>>> le32 next_off :15;
>>> le32 next_wrap : 1;
>>> ?  
>> Same thing in C, but would be more confusing IMHO since it will be up to
>> the reader to figure out which fields comprise the 32 bit integer.
> It looked weird to me. Other opinions?

Regarding the c11 standard the two are equivalent. Thus it does not
matter to me which notation is used. AFAIK bit-fields are only defined
in the context of structs (and/or unions), so I assumed that. Putting a
struct around it would be much better IMHO.

I don't agree with Michael's argument about 'which fields comprise the
32 bit integer', as IMHO it does not make sense in terms of c11.


struct A {
	uint32_t a:30, b:1, c:2:, d:8;

I think, in this particular case the notation ain't very helpful in
figuring out what comprises what. For that reason, if I really need
to choose, I would side with Connie on this one.

But there is another, more significant problem IMHO. The guarantees
provided by the C language (c11) regarding the resulting memory layout
are not sufficient to reason about it like Michael's comment and
the bit's of the draft imply. To know the memory layout we need the
ABI specification for the given platform on top of the C standard.

So if the bit fields are about in memory layout, I find the stuff
problematic. If however we use bit-fields only to define how arithmetic
works, then we are fine.

And a side note: A stronger/better reference to the C language could
benefit clarity. Especially if we start relying on the less trivial
properties of the C language.

Hope this helps!


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