Re: [Qemu-devel] [PATCHv3] tap: set IFF_ONE_QUEUE per default

[Michael S. Tsirkin]

On Mon, Feb 25, 2013 at 10:17:08AM +0100, Peter Lieven wrote:
> historically the kernel queues packets two times. once
> at the device and second in qdisc. this is believed to cause
> interface stalls if one of these queues overruns.
> 
> setting IFF_ONE_QUEUE is the default in kernels >= 3.8. the
> flag is ignored since then. see kernel commit
> 5d097109257c03a71845729f8db6b5770c4bbedc
> 
> v3:
>  - probe if IFF_ONE_QUEUE feature is available
> 
> v2:
>  - do only set the flag on linux as it breaks macvtap
>  - define IFF_ONE_QUEUE in tap-linux.h
> 
> Signed-off-by: Peter Lieven <address@hidden>


Acked-by: Michael S. Tsirkin <address@hidden>

> ---
>  net/tap-linux.c |   10 ++++++----
>  net/tap-linux.h |    9 +++++----
>  2 files changed, 11 insertions(+), 8 deletions(-)
> 
> diff --git a/net/tap-linux.c b/net/tap-linux.c
> index a953189..36c09e2 100644
> --- a/net/tap-linux.c
> +++ b/net/tap-linux.c
> @@ -42,6 +42,7 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
>      struct ifreq ifr;
>      int fd, ret;
>      int len = sizeof(struct virtio_net_hdr);
> +    unsigned int features;
> 
>      TFR(fd = open(PATH_NET_TUN, O_RDWR));
>      if (fd < 0) {
> @@ -51,9 +52,12 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
>      memset(&ifr, 0, sizeof(ifr));
>      ifr.ifr_flags = IFF_TAP | IFF_NO_PI;
> 
> -    if (*vnet_hdr) {
> -        unsigned int features;
> +    if (ioctl(fd, TUNGETFEATURES, &features) == 0 &&
> +        features & IFF_ONE_QUEUE) {
> +        ifr.ifr_flags |= IFF_ONE_QUEUE;
> +    }
> 
> +    if (*vnet_hdr) {
>          if (ioctl(fd, TUNGETFEATURES, &features) == 0 &&
>              features & IFF_VNET_HDR) {
>              *vnet_hdr = 1;
> @@ -78,8 +82,6 @@ int tap_open(char *ifname, int ifname_size, int *vnet_hdr,
>      }
> 
>      if (mq_required) {
> -        unsigned int features;
> -
>          if ((ioctl(fd, TUNGETFEATURES, &features) != 0) ||
>              !(features & IFF_MULTI_QUEUE)) {
>              error_report("multiqueue required, but no kernel "
> diff --git a/net/tap-linux.h b/net/tap-linux.h
> index 65087e1..5704efb 100644
> --- a/net/tap-linux.h
> +++ b/net/tap-linux.h
> @@ -34,10 +34,11 @@
>  #endif
> 
>  /* TUNSETIFF ifr flags */
> -#define IFF_TAP              0x0002
> -#define IFF_NO_PI    0x1000
> -#define IFF_VNET_HDR 0x4000
> -#define IFF_MULTI_QUEUE 0x0100
> +#define IFF_TAP          0x0002
> +#define IFF_NO_PI        0x1000
> +#define IFF_ONE_QUEUE    0x2000
> +#define IFF_VNET_HDR     0x4000
> +#define IFF_MULTI_QUEUE  0x0100
>  #define IFF_ATTACH_QUEUE 0x0200
>  #define IFF_DETACH_QUEUE 0x0400
> 
> -- 
> 1.7.9.5