Re: [PATCH] tap-linux: Open ipvtap and macvtap

[Akihiko Odaki <akihiko.odaki@daynix.com>]

On 2024/10/09 16:41, Jason Wang wrote:
> On Tue, Oct 8, 2024 at 2:52 PM Akihiko Odaki <akihiko.odaki@daynix.com> wrote:
>>
>> ipvtap and macvtap create a file for each interface unlike tuntap, which
>> creates one file shared by all interfaces. Try to open a file dedicated
>> to the interface first for ipvtap and macvtap.
>>
> 
> Management layers usually pass these fds via SCM_RIGHTS. Is this for
> testing purposes? (Note that we can use something like -netdev
> tap,fd=10 10<>/dev/tap0).

I used this for testing.

> 
>> Signed-off-by: Akihiko Odaki <akihiko.odaki@daynix.com>
>> ---
>>   net/tap-linux.c | 17 ++++++++++++++---
>>   1 file changed, 14 insertions(+), 3 deletions(-)
>>
>> diff --git a/net/tap-linux.c b/net/tap-linux.c
>> index 1226d5fda2d9..22ec2f45d2b7 100644
>> --- a/net/tap-linux.c
>> +++ b/net/tap-linux.c
>> @@ -45,10 +45,21 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
>>       int len = sizeof(struct virtio_net_hdr);
>>       unsigned int features;
>>
>> -    fd = RETRY_ON_EINTR(open(PATH_NET_TUN, O_RDWR));
>> +
>> +    ret = if_nametoindex(ifname);
>> +    if (ret) {
>> +        g_autofree char *file = g_strdup_printf("/dev/tap%d", ret);
>> +        fd = open(file, O_RDWR);
>> +    } else {
>> +        fd = -1;
>> +    }
>> +
>>       if (fd < 0) {
>> -        error_setg_errno(errp, errno, "could not open %s", PATH_NET_TUN);
>> -        return -1;
>> +        fd = RETRY_ON_EINTR(open(PATH_NET_TUN, O_RDWR));
> 
> Any reason tuntap were tried after the macvtap/ipvtap?

If we try tuntap first, we will know that it is not tuntap when calling 
TUNSETIFF. We will need to call TUNGETFEATURES and TUNSETVNETHDRSZ again 
in such a case because they precede TUNSETIFF. Calling them twice is 
troublesome.

This is also consistent with libvirt. libvirt first checks if 
g_strdup_printf("/dev/tap%d", ifindex) exists, and falls back to tuntap 
otherwise.

Regards,
Akihiko Odaki

> 
>> +        if (fd < 0) {
>> +            error_setg_errno(errp, errno, "could not open %s", PATH_NET_TUN);
>> +            return -1;
>> +        }
>>       }
>>       memset(&ifr, 0, sizeof(ifr));
>>       ifr.ifr_flags = IFF_TAP | IFF_NO_PI;
>>
>> ---
>> base-commit: 31669121a01a14732f57c49400bc239cf9fd505f
>> change-id: 20241008-macvtap-b152e5abb457
>>
>> Best regards,
>> --
>> Akihiko Odaki <akihiko.odaki@daynix.com>
> 
> Thanks
> 
>>
>