Re: [Qemu-devel] [PATCH] virtio-net: count VIRTIO_NET_F_MAC when calculating config_len

[Jesse Larrew]

On 04/25/2013 01:59 AM, Michael S. Tsirkin wrote:
> On Thu, Apr 25, 2013 at 02:21:29PM +0800, Jason Wang wrote:
>> Commit 14f9b664 (hw/virtio-net.c: set config size using host features) tries 
>> to
>> calculate config size based on the host features. But it forgets the
>> VIRTIO_NET_F_MAC were always set for qemu later. This will lead a zero config
>> len for virtio-net device when both VIRTIO_NET_F_STATUS and VIRTIO_NET_F_MQ 
>> were
>> disabled form command line. Then qemu will crash when user tries to read the
>> config of virtio-net.
>>
>> Fix this by counting VIRTIO_NET_F_MAC and make sure the config at least 
>> contains
>> the mac address.
>>
>> Cc: Jesse Larrew <address@hidden>
>> Signed-off-by: Jason Wang <address@hidden>
>> ---
>>  hw/net/virtio-net.c |    3 ++-
>>  1 files changed, 2 insertions(+), 1 deletions(-)
>>
>> diff --git a/hw/net/virtio-net.c b/hw/net/virtio-net.c
>> index 70c8fce..33a70ef 100644
>> --- a/hw/net/virtio-net.c
>> +++ b/hw/net/virtio-net.c
>> @@ -1264,7 +1264,8 @@ static void 
>> virtio_net_guest_notifier_mask(VirtIODevice *vdev, int idx,
>>  
>>  void virtio_net_set_config_size(VirtIONet *n, uint32_t host_features)
>>  {
>> -    int i, config_size = 0;
>> +    /* VIRTIO_NET_F_MAC can't be disabled from qemu side */
>> +    int i, config_size = feature_sizes[0].end;
> 
> This would be cleaner:
>       host_features |= (1 << VIRTIO_NET_F_MAC);
> 
> no need for a comment then.
> 

It seems to me that the real problem here is that host_features isn't
properly populated before virtio_net_set_config_size() is called. Looking
at virtio_device_init(), we can see why:

static int virtio_device_init(DeviceState *qdev)
{
    VirtIODevice *vdev = VIRTIO_DEVICE(qdev);
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(qdev);
    assert(k->init != NULL);
    if (k->init(vdev) < 0) {
        return -1;
    }
    virtio_bus_plug_device(vdev);
    return 0;
}

virtio_net_set_config_size() is currently being called as part of the
k->init call, but the host_features aren't properly setup until the device
is plugged into the bus using virtio_bus_plug_device().

After talking with mdroth, I think the proper way to fix this would be to
extend VirtioDeviceClass to include a calculate_config_size() method that
can be called at the appropriate time during device initialization like so:

static int virtio_device_init(DeviceState *qdev)
{
    VirtIODevice *vdev = VIRTIO_DEVICE(qdev);
    VirtioDeviceClass *k = VIRTIO_DEVICE_GET_CLASS(qdev);
    assert(k->init != NULL);
    if (k->init(vdev) < 0) {
        return -1;
    }
    virtio_bus_plug_device(vdev);
+   if (k->calculate_config_size && k->calculate_config_size(vdev) < 0) {
+       return -1;
+   }
    return 0;
}

This would ensure that host_features contains the proper data before any
devices try to make use of it to calculate the config size.

>>      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);
>> -- 
>> 1.7.1
> 

Jesse Larrew
Software Engineer, KVM Team
IBM Linux Technology Center
Phone: (512) 973-2052 (T/L: 363-2052)
address@hidden