[Qemu-devel] Re: [PATCH 33/41] virtio-net: port to vmstate

["Michael S. Tsirkin" <mst@redhat.com>]

On Wed, Dec 02, 2009 at 07:38:03PM +0100, Juan Quintela wrote:
> "Michael S. Tsirkin" <mst@redhat.com> wrote:
> > On Wed, Dec 02, 2009 at 01:04:31PM +0100, Juan Quintela wrote:
> >> 
> >> Signed-off-by: Juan Quintela <quintela@redhat.com>
> >> ---
> >>  hw/virtio-net.c |  148 ++++++++++++++++++++++++-------------------------------
> >>  1 files changed, 64 insertions(+), 84 deletions(-)
> >> 
> >> diff --git a/hw/virtio-net.c b/hw/virtio-net.c
> >> index 4434827..3a59449 100644
> >> --- a/hw/virtio-net.c
> >> +++ b/hw/virtio-net.c
> >> @@ -703,6 +703,38 @@ static void virtio_net_tx_timer(void *opaque)
> >>      virtio_net_flush_tx(n, n->tx_vq);
> >>  }
> >> 
> >> +/* Restore an uint8_t from an uint32_t
> >> +   This is a Big hack, but it is how the old state did it.
> >> + */
> >> +
> >> +static int get_uint8_from_uint32(QEMUFile *f, void *pv, size_t size)
> >> +{
> >> +    uint8_t *v = pv;
> >> +    *v = qemu_get_be32(f);
> >> +    return 0;
> >> +}
> >> +
> >> +static void put_unused(QEMUFile *f, void *pv, size_t size)
> >> +{
> >> +    fprintf(stderr, "uint8_from_uint32 is used only for backwards compatibility.\n");
> >
> > line too long
> >
> >> +    fprintf(stderr, "Never should be used to write a new state.\n");
> >> +    exit(0);
> >
> > I don't understand.  what is this dong?
> 
> it is used later.
> current code is reading an uint32_t value into a int8_t value.  As you
> can guess that don't fit (that is the HACK part of it).

I expect it fits in practice.
But you should range check the value and fail migration on error.

>  That is only
> needed for old versions that we are reading (get_* function has real
> code).  But we are supposed to never write old versions (*).
> Thet that shouldn't happen ever.

vmstate guarantees this won't be called?
So just assert(1)? Let's not write a ton of
code that isn't called?


> 
> 
> >  when
> > is this called? Please supply a comment.
> > Maybe call assert?
> >
> 
> assert or exit is ok for me, what does people preffer?
> 
> Later, Juan.
> 
> (*): My next series will propose to change that and allow to write old
>      versions, but that didn't exist when this code was written, and
>      there are still no agreement about how/if doing it.