Re: [Qemu-devel] [PATCH] virtio: right size for virtio_queue_get_avail_size

[Greg Kurz <gkurz@linux.vnet.ibm.com>]

On Wed, 2 Sep 2015 17:50:55 +0200
Cornelia Huck <cornelia.huck@de.ibm.com> wrote:

> On Wed,  2 Sep 2015 17:23:49 +0200
> Pierre Morel <pmorel@linux.vnet.ibm.com> wrote:
> 
> > Being working on dataplane I notice something strange:
> > 
> > virtio_queue_get_avail_size() used a 64bit size index
> > for the calculation of the available ring size.
> > 
> > It is quite strange but it did work with the old calculation
> > of the avail ring, at most with performance penalty,
> > and I wonder where I missed something.
> > 
> > This patch let use a 16bit size as defined in virtio_ring.h
> > 
> > Signed-off-by: Pierre Morel <pmorel@linux.vnet.ibm.com>
> > ---
> >  hw/virtio/virtio.c | 2 +-
> >  1 file changed, 1 insertion(+), 1 deletion(-)
> > 
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index 788b556..5c856eb 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -1460,7 +1460,7 @@ hwaddr virtio_queue_get_desc_size(VirtIODevice *vdev, int n)
> >  hwaddr virtio_queue_get_avail_size(VirtIODevice *vdev, int n)
> >  {
> >      return offsetof(VRingAvail, ring) +
> > -        sizeof(uint64_t) * vdev->vq[n].vring.num;
> > +        sizeof(uint16_t) * vdev->vq[n].vring.num;
> >  }
> > 
> >  hwaddr virtio_queue_get_used_size(VirtIODevice *vdev, int n)
> 
> I'm wondering about the semantics of the _size() functions. Naively I
> would expect (size of buffer) * (number of buffers). I think at least

Looking at where these functions are called, it really looks like they are
expected to return the size of the memory region to be mapped. Since we have:

typedef struct VRingAvail
{
    uint16_t flags;
    uint16_t idx;
    uint16_t ring[0];
} VRingAvail;

Pierre's patch looks valid. But while we're here, why not introducing
something like:

#define member_size(type, member) sizeof(((type *)0)->member)

It would consolidate the _size functions and the types they are referring to:

-        sizeof(uint64_t) * vdev->vq[n].vring.num;
+        member_size(VRingAvail, vring[0]) * vdev->vq[n].vring.num;

> vhost expects the {used,avail} indices in there as well? The
> s390-virtio code seems not to expect the indices to be contained in the
> size, though...