Re: [PATCH 4/7] hw/scsi/virtio-scsi: Use VIRTIO_SCSI_COMMON() macro

[Peter Maydell]

On Tue, 31 Oct 2023 at 13:18, Kevin Wolf <kwolf@redhat.com> wrote:
>
> Am 17.10.2023 um 16:01 hat Philippe Mathieu-Daudé geschrieben:
> > Access QOM parent with the proper QOM VIRTIO_SCSI_COMMON() macro.
> >
> > Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
> > ---
> >  hw/scsi/virtio-scsi.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> >
> > diff --git a/hw/scsi/virtio-scsi.c b/hw/scsi/virtio-scsi.c
> > index 45b95ea070..fa53f0902c 100644
> > --- a/hw/scsi/virtio-scsi.c
> > +++ b/hw/scsi/virtio-scsi.c
> > @@ -761,7 +761,7 @@ static void virtio_scsi_fail_cmd_req(VirtIOSCSIReq *req)
> >
> >  static int virtio_scsi_handle_cmd_req_prepare(VirtIOSCSI *s, VirtIOSCSIReq 
> > *req)
> >  {
> > -    VirtIOSCSICommon *vs = &s->parent_obj;
> > +    VirtIOSCSICommon *vs = VIRTIO_SCSI_COMMON(s);
> >      SCSIDevice *d;
> >      int rc;
>
> Why is a dynamic cast more "proper" than a static type-safe access, even
> more so in a hot I/O path?

I dunno, but it is absolutely the standard QOM style to use
the cast macro to do the downcast, rather than fishing about in
the struct for the parent_obj field. If QOM casts are getting
too expensive we should ideally fix them rather than adding
more and more code that takes some side route around them...

thanks
-- PMM