Re: [virtio-comment] Feedback for chapter 4

[Thomas Huth <thuth@linux.vnet.ibm.com>]

On Sun, 12 Jan 2014 13:24:09 +0200
"Michael S. Tsirkin" <mst@redhat.com> wrote:

> On Fri, Jan 10, 2014 at 01:49:49PM +0100, Thomas Huth wrote:
> > 
> > Here's my feedback for Virtio draft 01, chapter 4:
[...]
> > Chapter 4.3.2.*:
> > 
> > - In this chapter, the C-structs are marked with "__attribute__ ((packed));"
> >   which is just a GNU-C extension, as far as I know. In the other chapters,
> >   the structs are not marked with this string. So for consistency, I'd remove
> >   them here, too (and maybe state somewhere at the beginning of the spec
> >   that structs are considered to be without compiler padding inbetween)
> 
> I agree since C spec seems to allow compilers to add
> arbitrary padding anywhere in theory, we should mention all structures
> assume there's no padding.
> 
> And in most places, we don't really need the packing as
> fields are aligned properly so any sane compiler will just do the right
> thing.
> 
> Unfortunately CCW has several instances where any reasonable
> compiler would instroduce padding, I think just a note won't be enough
> since compiler-introduced issues are so hard to debug.
> So I think that instead of asking people to use compiler extensions to
> suppress padding, it's best just to add padding explicitly.
> 
> Cornelia, can we do this?
> For example
> 
> struct virtio_feature_desc {
> 	be32 features;
> 	u8 index;
> 	u8 padding[3]; /* Optional padding */
> };

Another possibility would be to abandon the C structs in the spec
completely and describe the memory layout of the buffers with a table,
something like this:

 +---------+------+----------+
 |  Offset | Size | Name     |
 +---------+------+----------+
 |    0    |   4  | features |
 +---------+------+----------+
 |    4    |   1  | index    |
 +---------+------+----------+

That would also be easier to read for those who don't know C and would
like to write a driver or device for virtio in another programming
language.

 Thomas