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

[Cornelia Huck <cohuck@redhat.com>]

On Wed, 7 Mar 2018 17:05:24 +0100
Halil Pasic <pasic@linux.vnet.ibm.com> wrote:

> 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.
> 
> Consider 
> 
> 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.

I'm not sure we should go down the C standard rabbit hole. People have
gotten lost in there.

If the clarification of what we mean by this notation (patch 1 + the
update sent later) is not enough, I'd rather prefer us to add a
clarifying sentence/diagram/... there. I was mainly bothered by the
change to the definition in this patch...