Hi,

I compared perf result using this patch and a patch changing tap_send_frames_passt() to:

static size_t tap_send_frames_passt(const struct ctx *c,
                                     const struct iovec *iov,
                                     size_t bufs_per_frame, size_t nframes)
{
         struct msghdr mh = {
                 .msg_iovlen = bufs_per_frame,
         };
         size_t buf_offset;
         unsigned int i;
         ssize_t sent;

         for (i = 0; i < nframes; i++) {
                 unsigned int j;

                 if (bufs_per_frame > 1) {
			/* if we have more than 1 iovec, the first one is vnet_len */
                         uint32_t *p = iov[i * bufs_per_frame].iov_base;
                         uint32_t vnet_len = 0;

                         for (j = 1; j < bufs_per_frame; j++)
                                 vnet_len += iov[i * bufs_per_frame + j].iov_len;
                         vnet_len = htonl(vnet_len);

                         *p = vnet_len;
                 }

                 mh.msg_iov = (void *)&iov[i * bufs_per_frame];

                 sent = sendmsg(c->fd_tap, &mh, MSG_NOSIGNAL | MSG_DONTWAIT);
                 if (sent < 0)
                         return i;

                 /* Check for any partial frames due to short send */
                 j = iov_skip_bytes(&iov[i * bufs_per_frame], bufs_per_frame, sent, 
&buf_offset);

                 if (buf_offset && j < bufs_per_frame) {
                         if (write_remainder(c->fd_tap, &iov[i * bufs_per_frame + j],
                                             bufs_per_frame - j,
                                             buf_offset) < 0) {
                                 err("tap: partial frame send: %s",
                                     strerror(errno));
                                 return i;
                         }
                 }
         }

         return i;
}

And the result of 'perf record -e cache-misses' gives:

slow

   83.95%  passt.avx2  passt.avx2            [.] csum_avx2
    4.39%  passt.avx2  passt.avx2            [.] tap4_handler
    2.37%  passt.avx2  libc.so.6             [.] __printf_buffer
    0.84%  passt.avx2  passt.avx2            [.] udp_timer

fast

     22.15%  passt.avx2  passt.avx2            [.] csum_avx2
     14.91%  passt.avx2  passt.avx2            [.] udp_timer
      7.60%  passt.avx2  libc.so.6             [.] __printf_buffer
      5.10%  passt.avx2  passt.avx2            [.] ffsl

Thanks,
Laurent