Re: [Qemu-devel] [RFC v2 3/3] virtio-net: Add MTU feature support

[Michael S. Tsirkin]

On Tue, Nov 22, 2016 at 01:11:50PM +0100, Maxime Coquelin wrote:
> 
> 
> On 11/21/2016 05:48 PM, Michael S. Tsirkin wrote:
> > > > > > > > @@ -1695,6 +1706,7 @@ static void 
> > > > > > > > virtio_net_set_config_size(VirtIONet *n, uint64_t host_features)
> > > > > > > >  {
> > > > > > > >      int i, config_size = 0;
> > > > > > > >      virtio_add_feature(&host_features, VIRTIO_NET_F_MAC);
> > > > > > > > +    virtio_add_feature(&host_features, VIRTIO_NET_F_MTU);
> > > > > > > >      for (i = 0; feature_sizes[i].flags != 0; i++) {
> > > > > > > >          if (host_features & feature_sizes[i].flags) {
> > > > > > > >              config_size = MAX(feature_sizes[i].end, 
> > > > > > > > config_size);
> > > > > > > > @@ -1922,6 +1934,8 @@ static Property virtio_net_properties[] = 
> > > > > > > > {
> > > > > > > >      DEFINE_PROP_STRING("tx", VirtIONet, net_conf.tx),
> > > > > > > >      DEFINE_PROP_UINT16("rx_queue_size", VirtIONet, 
> > > > > > > > net_conf.rx_queue_size,
> > > > > > > >                         VIRTIO_NET_RX_QUEUE_DEFAULT_SIZE),
> > > > > > > > +    DEFINE_PROP_BIT("host_mtu", VirtIONet, host_features,
> > > > > > > > +                    VIRTIO_NET_F_MTU, true),
> > > > > > > >      DEFINE_PROP_END_OF_LIST(),
> > > > > > > >  };
> > > > > > > >
> > > > > >
> > > > > > Cross version migration support is missing here.
> > > > Sorry, I'm not sure to understand what you expect here.
> > > > Could you please provide more details?
> > feature bits must be consistent for a given machine type.
> > So you can't add them unconditionally for old machine
> > types, they must look the same as they looked when
> > we put that machine type out.
> 
> Ok, thanks for clarifying.
> IIUC, idea is to let it enabled by default, and add entries in
> HW_COMPAT_2_x macros to disable it in all previous versions?
> 
> Thanks,
> Maxime

I suspect you should keep it disabled by default,
set it when user specifies the mtu.

-- 
MST