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

[Stefano Stabellini]

On Tue, 25 Nov 2014, Jason Wang wrote:
> On 11/25/2014 02:44 AM, 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.
> >
> > 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));
> 
> Does this really work?

It seems to work here, as in it doesn't crash QEMU and I am able to boot
a guest with network. I didn't try any MAC related commands.


> The code in fact skips the location that contains
> virtio_net_ctrl_hdr. And virtio_net_handle_mac() still calls
> iov_discard_front().
>
> How about copy iov to a temp variable and use it in this function?

That would only work if I moved the cpu_physical_memory_unmap call
outside of virtqueue_fill, so that we can pass different iov to them.
We need to unmap the same iov that was previously mapped by
virtqueue_pop.