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Date: Wed, 02 Nov 2011 11:12:03 -0400
From: Corey Bryant <coreyb@linux.vnet.ibm.com>
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Subject: Re: [Qemu-devel] [PATCH v4 1/4] Add basic version of bridge helper
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To: Stefan Hajnoczi <stefanha@gmail.com>
Cc: rmarwah@linux.vnet.ibm.com, aliguori@us.ibm.com, qemu-devel@nongnu.org

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