Re: [Qemu-devel] [RFC 16/29] vhost+postcopy: Stash RAMBlock and offset

[Peter Xu <peterx@redhat.com>]

On Fri, Jul 14, 2017 at 06:15:54PM +0100, Dr. David Alan Gilbert wrote:
> * Peter Xu (peterx@redhat.com) wrote:
> > On Wed, Jun 28, 2017 at 08:00:34PM +0100, Dr. David Alan Gilbert (git) wrote:
> > > From: "Dr. David Alan Gilbert" <dgilbert@redhat.com>
> > > 
> > > Stash the RAMBlock and offset for later use looking up
> > > addresses.
> > > 
> > > Signed-off-by: Dr. David Alan Gilbert <dgilbert@redhat.com>
> > > ---
> > >  hw/virtio/trace-events |  1 +
> > >  hw/virtio/vhost-user.c | 11 +++++++++++
> > >  2 files changed, 12 insertions(+)
> > > 
> > > diff --git a/hw/virtio/trace-events b/hw/virtio/trace-events
> > > index f7be340a45..1fd194363a 100644
> > > --- a/hw/virtio/trace-events
> > > +++ b/hw/virtio/trace-events
> > > @@ -3,6 +3,7 @@
> > >  # hw/virtio/vhost-user.c
> > >  vhost_user_postcopy_listen(void) ""
> > >  vhost_user_set_mem_table_postcopy(uint64_t client_addr, uint64_t qhva, int reply_i, int region_i) "client:%"PRIx64" for hva: %"PRIx64" reply %d region %d"
> > > +vhost_user_set_mem_table_withfd(int index, const char *name, uint64_t memory_size, uint64_t guest_phys_addr, uint64_t userspace_addr, uint64_t offset) "%d:%s: size:%"PRIx64" GPA:%"PRIx64" QVA/userspace:%"PRIx64" RB offset:%"PRIx64
> > >  
> > >  # hw/virtio/virtio.c
> > >  virtqueue_alloc_element(void *elem, size_t sz, unsigned in_num, unsigned out_num) "elem %p size %zd in_num %u out_num %u"
> > > diff --git a/hw/virtio/vhost-user.c b/hw/virtio/vhost-user.c
> > > index 6be3e7ff2d..3185af7a45 100644
> > > --- a/hw/virtio/vhost-user.c
> > > +++ b/hw/virtio/vhost-user.c
> > > @@ -133,6 +133,11 @@ struct vhost_user {
> > >      NotifierWithReturn postcopy_notifier;
> > >      struct PostCopyFD  postcopy_fd;
> > >      uint64_t           postcopy_client_bases[VHOST_MEMORY_MAX_NREGIONS];
> > > +    RAMBlock          *region_rb[VHOST_MEMORY_MAX_NREGIONS];
> > > +    /* The offset from the start of the RAMBlock to the start of the
> > > +     * vhost region.
> > > +     */
> > > +    ram_addr_t         region_rb_offset[VHOST_MEMORY_MAX_NREGIONS];
> > 
> > Here the array size is VHOST_MEMORY_MAX_NREGIONS, while...
> > 
> > >  };
> > >  
> > >  static bool ioeventfd_enabled(void)
> > > @@ -324,8 +329,14 @@ static int vhost_user_set_mem_table(struct vhost_dev *dev,
> > >          assert((uintptr_t)reg->userspace_addr == reg->userspace_addr);
> > >          mr = memory_region_from_host((void *)(uintptr_t)reg->userspace_addr,
> > >                                       &offset);
> > > +        u->region_rb_offset[i] = offset;
> > > +        u->region_rb[i] = mr->ram_block;
> > 
> > ... can i>=VHOST_MEMORY_MAX_NREGIONS here? Or do we only need to note
> > this down if fd > 0 below?  Thanks,
> 
> I don't *think* so - I mean:
>     for (i = 0; i < dev->mem->nregions; ++i) {
> 
> so if that's the maximum number of regions and that's the number of
> regions we should be safe???

That's my concern - looks like dev->mem->nregions can be bigger than
that? At least I didn't really see a restriction on its size. The size
is changed in following stack:

  vhost_region_add
    vhost_set_memory
      vhost_dev_assign_memory

And it's dynamic extended, without checks.

Indeed in the function vhost_user_set_mem_table() we have:

    int fds[VHOST_MEMORY_MAX_NREGIONS];

But we are safe iiuc because we also have assertions to protect:

    assert(fd_num < VHOST_MEMORY_MAX_NREGIONS);
    fds[fd_num++] = fd;

Do we at least need that assert?

Thanks,

-- 
Peter Xu