Re: [Qemu-devel] [Xen-devel] virtio leaks cpu mappings, was: qemu crash with virtio on Xen domUs (backtrace included)

[Stefano Stabellini]

On Mon, 24 Nov 2014, Konrad Rzeszutek Wilk wrote:
> On Mon, Nov 24, 2014 at 06:44:45PM +0000, Stefano Stabellini wrote:
> > On Mon, 24 Nov 2014, Stefano Stabellini wrote:
> > > On Mon, 24 Nov 2014, Stefano Stabellini wrote:
> > > > CC'ing Paolo.
> > > > 
> > > > 
> > > > Wen,
> > > > thanks for the logs.
> > > > 
> > > > I investigated a little bit and it seems to me that the bug occurs when
> > > > QEMU tries to unmap only a portion of a memory region previously mapped.
> > > > That doesn't work with xen-mapcache.
> > > > 
> > > > See these logs for example:
> > > > 
> > > > DEBUG address_space_map phys_addr=78ed8b44 vaddr=7fab50afbb68 len=0xa
> > > > DEBUG address_space_unmap vaddr=7fab50afbb68 len=0x6
> > > 
> > > Sorry the logs don't quite match, it was supposed to be:
> > > 
> > > DEBUG address_space_map phys_addr=78ed8b44 vaddr=7fab50afbb64 len=0xa
> > > DEBUG address_space_unmap vaddr=7fab50afbb68 len=0x6
> > 
> > It looks like the problem is caused by iov_discard_front, called by
> > virtio_net_handle_ctrl. By changing iov_base after the sg has already
> > been mapped (cpu_physical_memory_map), it causes a leak in the mapping
> > because the corresponding cpu_physical_memory_unmap will only unmap a
> > portion of the original sg.  On Xen the problem is worse because
> > xen-mapcache aborts.
> 
> Didn't um Andy post patches for ths:
> 
> http://lists.xen.org/archives/html/xen-devel/2014-09/msg02864.html

It looks like these are fixing a linux side issue (the frontend) while
this patch is for a bug in QEMU (the backend).


> > 
> > diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
> > index 2ac6ce5..b2b5c2d 100644
> > --- a/hw/net/virtio-net.c
> > +++ b/hw/net/virtio-net.c
> > @@ -775,7 +775,7 @@ static void virtio_net_handle_ctrl(VirtIODevice *vdev, 
> > VirtQueue *vq)
> >      struct iovec *iov;
> >      unsigned int iov_cnt;
> >  
> > -    while (virtqueue_pop(vq, &elem)) {
> > +    while (virtqueue_pop_nomap(vq, &elem)) {
> >          if (iov_size(elem.in_sg, elem.in_num) < sizeof(status) ||
> >              iov_size(elem.out_sg, elem.out_num) < sizeof(ctrl)) {
> >              error_report("virtio-net ctrl missing headers");
> > @@ -784,8 +784,12 @@ static void virtio_net_handle_ctrl(VirtIODevice *vdev, 
> > VirtQueue *vq)
> >  
> >          iov = elem.out_sg;
> >          iov_cnt = elem.out_num;
> > -        s = iov_to_buf(iov, iov_cnt, 0, &ctrl, sizeof(ctrl));
> >          iov_discard_front(&iov, &iov_cnt, sizeof(ctrl));
> > +
> > +        virtqueue_map_sg(elem.in_sg, elem.in_addr, elem.in_num, 1);
> > +        virtqueue_map_sg(elem.out_sg, elem.out_addr, elem.out_num, 0);
> > +
> > +        s = iov_to_buf(iov, iov_cnt, 0, &ctrl, sizeof(ctrl));
> >          if (s != sizeof(ctrl)) {
> >              status = VIRTIO_NET_ERR;
> >          } else if (ctrl.class == VIRTIO_NET_CTRL_RX) {
> > diff --git a/hw/virtio/virtio.c b/hw/virtio/virtio.c
> > index 3e4b70c..6a4bd3a 100644
> > --- a/hw/virtio/virtio.c
> > +++ b/hw/virtio/virtio.c
> > @@ -446,7 +446,7 @@ void virtqueue_map_sg(struct iovec *sg, hwaddr *addr,
> >      }
> >  }
> >  
> > -int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)
> > +int virtqueue_pop_nomap(VirtQueue *vq, VirtQueueElement *elem)
> >  {
> >      unsigned int i, head, max;
> >      hwaddr desc_pa = vq->vring.desc;
> > @@ -505,9 +505,6 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)
> >          }
> >      } while ((i = virtqueue_next_desc(desc_pa, i, max)) != max);
> >  
> > -    /* Now map what we have collected */
> > -    virtqueue_map_sg(elem->in_sg, elem->in_addr, elem->in_num, 1);
> > -    virtqueue_map_sg(elem->out_sg, elem->out_addr, elem->out_num, 0);
> >  
> >      elem->index = head;
> >  
> > @@ -517,6 +514,16 @@ int virtqueue_pop(VirtQueue *vq, VirtQueueElement 
> > *elem)
> >      return elem->in_num + elem->out_num;
> >  }
> >  
> > +int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem)
> > +{
> > +    int rc = virtqueue_pop_nomap(vq, elem);
> > +    if (rc > 0) {
> > +        virtqueue_map_sg(elem->in_sg, elem->in_addr, elem->in_num, 1);
> > +        virtqueue_map_sg(elem->out_sg, elem->out_addr, elem->out_num, 0);
> > +    }
> > +    return rc;
> > +}
> > +
> >  /* virtio device */
> >  static void virtio_notify_vector(VirtIODevice *vdev, uint16_t vector)
> >  {
> > diff --git a/include/hw/virtio/virtio.h b/include/hw/virtio/virtio.h
> > index 3e54e90..40a3977 100644
> > --- a/include/hw/virtio/virtio.h
> > +++ b/include/hw/virtio/virtio.h
> > @@ -174,6 +174,7 @@ void virtqueue_fill(VirtQueue *vq, const 
> > VirtQueueElement *elem,
> >  void virtqueue_map_sg(struct iovec *sg, hwaddr *addr,
> >      size_t num_sg, int is_write);
> >  int virtqueue_pop(VirtQueue *vq, VirtQueueElement *elem);
> > +int virtqueue_pop_nomap(VirtQueue *vq, VirtQueueElement *elem);
> >  int virtqueue_avail_bytes(VirtQueue *vq, unsigned int in_bytes,
> >                            unsigned int out_bytes);
> >  void virtqueue_get_avail_bytes(VirtQueue *vq, unsigned int *in_bytes,
> > 
> > _______________________________________________
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>