Re: [Qemu-devel] [SeaBIOS] [PATCH 07/18] virtio: add version 1.0 read/write macros

[Gerd Hoffmann <kraxel@redhat.com>]

On Mo, 2015-06-29 at 09:02 -0400, Kevin O'Connor wrote:
> On Mon, Jun 29, 2015 at 10:53:29AM +0200, Gerd Hoffmann wrote:
> > Add macros to read/write registers of virtio-1.0 regions.
> > 
> > Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
> > ---
> >  src/hw/virtio-pci.h | 76 +++++++++++++++++++++++++++++++++++++++++++++++++++++
> >  1 file changed, 76 insertions(+)
> > 
> > diff --git a/src/hw/virtio-pci.h b/src/hw/virtio-pci.h
> > index 893a7dd..e1d8b3e 100644
> > --- a/src/hw/virtio-pci.h
> > +++ b/src/hw/virtio-pci.h
> > @@ -111,6 +111,82 @@ struct vp_device {
> >      struct vp_cap common, notify, isr, device;
> >  };
> >  
> > +#define vp_modern_read(_cap, _struct, _field, _var) {                   \
> > +        u32 addr = _cap.addr;                                           \
> > +        addr += offsetof(_struct, _field);                              \
> 
> Wouldn't this make more sense if the bulk of the code was in a
> function?

The idea is that 'sizeof((_struct *)0)->_field)' evaluates to a
compile-time constant, so gcc can optimize away the bulk of the #define.
Also we don't have to use u64 for _var then (except when it actually is
a 64bit value).

But having 'vp_modern_read(..., var)' in the code instead of 'var =
vp_modern_read(...)' isn't that nice indeed.

> > +        if (_cap.is_io) {                                               \
> > +            switch (sizeof(((_struct *)0)->_field)) {                   \
> > +            case 8:                                                     \
> > +                _var = inl(addr);                                       \
> > +                _var |= (u64)inl(addr+4) << 32;                         \
> > +                break;                                                  \
> 
> It didn't look like there were any 64bit fields defined, but maybe I
> missed something.

The is a single one (number of sectors for virtio-blk disks).

cheers,
  Gerd