[PATCH RFC] virtio-pci: new config layout: using memory BAR

Michael S. Tsirkin
Thu, 30 May 2013 07:55:24 -0000

On Thu, May 30, 2013 at 01:28:26PM +0930, Rusty Russell
wrote:
> >> Yet the structure definitions are descriptive, capturing layout,
size
> >> and endianness in natural a format readable by any C programmer.
> >
> >>From an API design point of view, here are the problems I see:
> >
> > 1) C makes no guarantees about structure layout beyond the first
> >    member.  Yes, if it's naturally aligned or has a packed
attribute,
> >    GCC does the right thing.  But this isn't kernel land anymore,
> >    portability matters and there are more compilers than GCC.
>
> [ I argued in detail here, then deleted.  Damn bikeshedding... ]
>
> I think the best thing is to add the decoded integer versions as macros,
> and have a heap of BUILD_BUG_ON() in the kernel source to make sure they
> match.

Hmm you want to have both in the header?

That would confuse users for sure :)

I guess we could put in a big comment explaning
why do we have both, and which version should
userspace use. I tried to write it:

/*
 * On all known C compilers, you can use the
 * offsetof macro to find the correct offset of each field -
 * if your compiler doesn't implement this correctly, use the
 * integer versions below, instead.
 * In that case please don't use the structures above, to avoid confusion.
 */


--
MST