On Tue, 4 Jun 2019 10:06:36 +0200
Björn Töpel [off-list ref] wrote:

On 2019-06-03 15:19, Maciej Fijalkowski wrote:

quoted

Let's get into the driver via ndo_bpf with command set to XDP_SETUP_UMEM
with fill queue that already contains some available entries that can be
used by Rx driver rings. Things worked in such way on old version of
xdpsock (that lacked libbpf support) and there's no particular reason
for having this preparation done after bind().

Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Krzysztof Kazimierczak <redacted>
---
  samples/bpf/xdpsock_user.c | 15 ---------------
  tools/lib/bpf/xsk.c        | 19 ++++++++++++++++++-
  2 files changed, 18 insertions(+), 16 deletions(-)

diff --git a/samples/bpf/xdpsock_user.c b/samples/bpf/xdpsock_user.c
index d08ee1ab7bb4..e9dceb09b6d1 100644
--- a/samples/bpf/xdpsock_user.c
+++ b/samples/bpf/xdpsock_user.c

@@ -296,8 +296,6 @@ static struct xsk_socket_info *xsk_configure_socket(struct xsk_umem_info *umem)
  	struct xsk_socket_config cfg;
  	struct xsk_socket_info *xsk;
  	int ret;
-	u32 idx;
-	int i;
  
  	xsk = calloc(1, sizeof(*xsk));
  	if (!xsk)

@@ -318,19 +316,6 @@ static struct xsk_socket_info *xsk_configure_socket(struct xsk_umem_info *umem)
  	if (ret)
  		exit_with_error(-ret);
  
-	ret = xsk_ring_prod__reserve(&xsk->umem->fq,
-				     XSK_RING_PROD__DEFAULT_NUM_DESCS,
-				     &idx);
-	if (ret != XSK_RING_PROD__DEFAULT_NUM_DESCS)
-		exit_with_error(-ret);
-	for (i = 0;
-	     i < XSK_RING_PROD__DEFAULT_NUM_DESCS *
-		     XSK_UMEM__DEFAULT_FRAME_SIZE;
-	     i += XSK_UMEM__DEFAULT_FRAME_SIZE)
-		*xsk_ring_prod__fill_addr(&xsk->umem->fq, idx++) = i;
-	xsk_ring_prod__submit(&xsk->umem->fq,
-			      XSK_RING_PROD__DEFAULT_NUM_DESCS);
-
  	return xsk;
  }
  

diff --git a/tools/lib/bpf/xsk.c b/tools/lib/bpf/xsk.c
index 38667b62f1fe..57dda1389870 100644
--- a/tools/lib/bpf/xsk.c
+++ b/tools/lib/bpf/xsk.c

@@ -529,7 +529,8 @@ int xsk_socket__create(struct xsk_socket **xsk_ptr, const char *ifname,
  	struct xdp_mmap_offsets off;
  	struct xsk_socket *xsk;
  	socklen_t optlen;
-	int err;
+	int err, i;
+	u32 idx;
  
  	if (!umem || !xsk_ptr || !rx || !tx)
  		return -EFAULT;

@@ -632,6 +633,22 @@ int xsk_socket__create(struct xsk_socket **xsk_ptr, const char *ifname,
  	}
  	xsk->tx = tx;
  
+	err = xsk_ring_prod__reserve(umem->fill,
+				     XSK_RING_PROD__DEFAULT_NUM_DESCS,
+				     &idx);
+	if (err != XSK_RING_PROD__DEFAULT_NUM_DESCS) {
+		err = -errno;
+		goto out_mmap_tx;
+	}
+
+	for (i = 0;
+	     i < XSK_RING_PROD__DEFAULT_NUM_DESCS *
+		     XSK_UMEM__DEFAULT_FRAME_SIZE;
+	     i += XSK_UMEM__DEFAULT_FRAME_SIZE)
+		*xsk_ring_prod__fill_addr(umem->fill, idx++) = i;
+	xsk_ring_prod__submit(umem->fill,
+			      XSK_RING_PROD__DEFAULT_NUM_DESCS);
+

Here, entries are added to the umem fill ring regardless if Rx is being
used or not. For a Tx only setup, this is not what we want, right?

Right, but we have such behavior even without this patch. So I see two options
here:
- if you agree with this patch, then I guess we would need to pass the info to
  libbpf what exactly we are setting up (txonly, rxdrop, l2fwd)?
- otherwise, we should be passing the opt_bench onto xsk_configure_socket and
  based on that decide whether we fill the fq or not?

Thinking out loud here; Now libbpf is making the decision which umem
entries that are added to the fill ring. The sample application has this
(naive) scheme. I'm not sure that all applications would like that
policy. What do you think?

I find it convenient to have the fill queue in "initialized" state if I am
making use of it, especially in case when I am doing the ZC so I must give the
buffers to the driver via fill queue. So why would we bother other applications
to provide it? I must admit that I haven't used AF_XDP with other apps than the
example one, so I might not be able to elaborate further. Maybe other people
have different feelings about it.

quoted

  	sxdp.sxdp_family = PF_XDP;
  	sxdp.sxdp_ifindex = xsk->ifindex;
  	sxdp.sxdp_queue_id = xsk->queue_id;