Re: [Qemu-devel] [PATCH v4 1/4] Add basic version of bridge helper

[Corey Bryant <coreyb@linux.vnet.ibm.com>]

On 11/02/2011 06:58 AM, Stefan Hajnoczi wrote:
> On Tue, Nov 1, 2011 at 5:13 PM, Corey Bryant<coreyb@linux.vnet.ibm.com>  wrote:
>> +static bool has_vnet_hdr(int fd)
>> +{
>> +    unsigned int features = 0;
>> +    struct ifreq ifreq;
>> +
>> +    if (ioctl(fd, TUNGETFEATURES,&features) == -1) {
>> +        return false;
>> +    }
>> +
>> +    if (!(features&  IFF_VNET_HDR)) {
>> +        return false;
>> +    }
>> +
>> +    if (ioctl(fd, TUNGETIFF,&ifreq) != -1 || errno != EBADFD) {
>> +        return false;
>> +    }
>
> I don't understand this expression.  We want TUNGETIFF to fail with
> EBADFD, otherwise we return false.  What is this trying to do?
>
> Why do we even need TUNGETIFF after TUNGETFEATURES succeeded and we
> were able to check out the IFF_VNET_HDR flag?
>
> Stefan
>

I don't think the TUNGETIFF call is necessary.  It verifies that the tap 
device doesn't already exist.  The ensuing TUNSETIFF call should already 
cover this.

Regards,
Corey