From mboxrd@z Thu Jan  1 00:00:00 1970
From: "Michael S. Tsirkin" <mst@redhat.com>
Subject: Re: [PATCH RFC] virtio-pci: new config layout: using memory BAR
Date: Wed, 29 May 2013 11:05:47 +0300
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To: Paolo Bonzini <pbonzini@redhat.com>
Cc: Peter Maydell <peter.maydell@linaro.org>, Anthony Liguori <aliguori@us.ibm.com>, kvm@vger.kernel.org, virtualization@lists.linux-foundation.org, Stefan Hajnoczi <stefanha@redhat.com>, Laszlo Ersek <lersek@redhat.com>, KONRAD Frederic <fred.konrad@greensocs.com>
List-Id: virtualization@lists.linuxfoundation.org

On Wed, May 29, 2013 at 09:27:02AM +0200, Paolo Bonzini wrote:
> Il 29/05/2013 06:33, Rusty Russell ha scritto:
> > Paolo Bonzini <pbonzini@redhat.com> writes:
> >> Il 28/05/2013 19:32, Michael S. Tsirkin ha scritto:
> >>>>>>> +
> >>>>>>> +    switch (addr) {
> >>>>>>> +    case offsetof(struct virtio_pci_common_cfg, device_feature_select):
> >>>>>>> +        return proxy->device_feature_select;
> >>>>>
> >>>>> Oh dear no...  Please use defines like the rest of QEMU.
> >>> Any good reason not to use offsetof?
> >>
> >> I'm not sure it's portable to use it in case labels.  IIRC, the
> >> definition ((int)&(((T *)0)->field)) is not a valid C integer constant
> >> expression.  Laszlo?
> > 
> > It's defined to yield an integer constant expression in the ISO standard
> > (and I think ANSI too, though that's not at hand):
> 
> It's not in C89.  The oldest compiler QEMU cares about is GCC 4.2.  I
> don't know if it has a builtin offsetof, probably it does.

Yes, I think __builtin_offsetof was added in 4.0:
http://gcc.gnu.org/onlinedocs/gcc-4.0.0/gcc/Offsetof.html


> But I'm not sure about other users of virtio headers.
> 
> Paolo
> > 
> >     7.19, para 3:
> > 
> >         ...offsetof(type, member-designator)
> >         which expands to an integer constant expression that has type
> >         size_t, ...
> > 
> > The real question is whether compilers qemu cares about meet the
> > standard (there's some evidence that older compilers fail this).  If
> > not, we'll have to define them as raw offsets...

So I think the question is whether we care about GCC 3.  We used to when
mingw in debian stable was GCC 3, but not anymore ...

> > 
> > Cheers,
> > Rusty.
> >