Re: [PATCH] virtio: remove virtio_tswap16s() call in vring_packed_event_read()

["Michael S. Tsirkin" <mst@redhat.com>]

On Mon, Jul 01, 2024 at 11:53:34AM +0200, Eugenio Perez Martin wrote:
> On Mon, Jul 1, 2024 at 9:52 AM Stefano Garzarella <sgarzare@redhat.com> wrote:
> >
> > Commit d152cdd6f6 ("virtio: use virtio accessor to access packed event")
> > switched using of address_space_read_cached() to virito_lduw_phys_cached()
> > to access packed descriptor event.
> >
> > When we used address_space_read_cached(), we needed to call
> > virtio_tswap16s() to handle the endianess of the field, but
> > virito_lduw_phys_cached() already handles it internally, so we no longer
> > need to call virtio_tswap16s() (as the commit had done for `off_wrap`,
> > but forgot for `flags`).
> >
> > Fixes: d152cdd6f6 ("virtio: use virtio accessor to access packed event")
> > Cc: jasowang@redhat.com
> > Cc: qemu-stable@nongnu.org
> > Reported-by: Xoykie <xoykie@gmail.com>
> > Link: https://lore.kernel.org/qemu-devel/CAFU8RB_pjr77zMLsM0Unf9xPNxfr_--Tjr49F_eX32ZBc5o2zQ@mail.gmail.com
> > Signed-off-by: Stefano Garzarella <sgarzare@redhat.com>
> 
> Reviewed-by: Eugenio Pérez <eperezma@redhat.com>
> 
> I think it would be great to test the patches using a big endian host
> just in case.
> 
> Thanks!

I think we really should move to using sparse tags for endian-ness,
like Linux does, and away from swap calls.


> > ---
> >  hw/virtio/virtio.c | 1 -
> >  1 file changed, 1 deletion(-)
> >
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index 893a072c9d..2e5e67bdb9 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -323,7 +323,6 @@ static void vring_packed_event_read(VirtIODevice *vdev,
> >      /* Make sure flags is seen before off_wrap */
> >      smp_rmb();
> >      e->off_wrap = virtio_lduw_phys_cached(vdev, cache, off_off);
> > -    virtio_tswap16s(vdev, &e->flags);
> >  }
> >
> >  static void vring_packed_off_wrap_write(VirtIODevice *vdev,
> > --
> > 2.45.2
> >