Re: [Qemu-devel] [PATCH v6 04/12] dataplane: add virtqueue vring code

[Stefan Hajnoczi]

On Tue, Dec 11, 2012 at 04:18:20PM +0200, Michael S. Tsirkin wrote:
> On Mon, Dec 10, 2012 at 02:09:37PM +0100, Stefan Hajnoczi wrote:
> > +        iov->iov_base = hostmem_lookup(&vring->hostmem, desc.addr, 
> > desc.len,
> > +                                       desc.flags & VRING_DESC_F_WRITE);
> > +        if (!iov->iov_base) {
> > +            error_report("Failed to map indirect descriptor"
> > +                         "addr %#" PRIx64 " len %u",
> > +                         (uint64_t)desc.addr, desc.len);
> > +            vring->broken = true;
> > +            return -EFAULT;
> > +        }
> > +        iov->iov_len = desc.len;
> > +        iov++;
> 
> Hmm, this assumes a descriptor can not cross a memory
> region boundary. Is this really guaranteed?
> vhost does not make such assumptions.

Just responded in the other subthread.

hw/virtio.c doesn't split across memory region boundaries.  This suggest
we don't hit this in practice.  If it does we'll print the error and
disable the device until virtio reset.

I can add it to my TODO list.  hw/virtio.c also uses exit(1) for
guest-triggerable errors at runtime and I'd like it to do something like
vring->broken above.

IMO it's not worth trying to perfect these things here.  The next step
after this series should be looking at thread-safe memory API so that we
can use hw/virtio.c and improve it for all users.

Stefan