[src/trunk]: src/sys/arch/xen/xen xennetback(4): Fix xennetback_evthandler loop.

[riastradh <riastradh%NetBSD.org@localhost>]

details:   https://anonhg.NetBSD.org/src/rev/9987aaf6694a
branches:  trunk
changeset: 373719:9987aaf6694a
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Sat Feb 25 00:34:13 2023 +0000

description:
xennetback(4): Fix xennetback_evthandler loop.

- After observing the other side has produced pending tx requests by
  reading sring->req_prod, must issue xen_rmb before touching them.

  Despite all the effort to use the heavy-weight
  RING_FINAL_CHECK_FOR_REQUESTS on each request in the loop, this
  barrier was missing.

- No need to update req_cons at each iteration in the loop.  It's
  private.  Just update it once at the end.

- After consuming requests, must issue xen_wmb before releasing the
  slots with RING_FINAL_CHECK_FOR_REQUEST for the other side to
  reuse.

XXX pullup-8 (requires patch; at least add xen_rmb between
    RING_FINAL_CHECK_FOR_REQUESTS and RING_COPY_REQUEST)
XXX pullup-9 (requires patch; at least add xen_rmb between
    RING_FINAL_CHECK_FOR_REQUESTS and RING_COPY_REQUEST)
XXX pullup-10

diffstat:

 sys/arch/xen/xen/xennetback_xenbus.c |  25 +++++++++++++------------
 1 files changed, 13 insertions(+), 12 deletions(-)

diffs (62 lines):

diff -r c416f9775bfe -r 9987aaf6694a sys/arch/xen/xen/xennetback_xenbus.c
--- a/sys/arch/xen/xen/xennetback_xenbus.c      Sat Feb 25 00:34:01 2023 +0000
+++ b/sys/arch/xen/xen/xennetback_xenbus.c      Sat Feb 25 00:34:13 2023 +0000
@@ -1,4 +1,4 @@
-/*      $NetBSD: xennetback_xenbus.c,v 1.108 2022/09/02 23:48:10 thorpej Exp $      */
+/*      $NetBSD: xennetback_xenbus.c,v 1.109 2023/02/25 00:34:13 riastradh Exp $      */
 
 /*
  * Copyright (c) 2006 Manuel Bouyer.
@@ -25,7 +25,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: xennetback_xenbus.c,v 1.108 2022/09/02 23:48:10 thorpej Exp $");
+__KERNEL_RCSID(0, "$NetBSD: xennetback_xenbus.c,v 1.109 2023/02/25 00:34:13 riastradh Exp $");
 
 #include <sys/types.h>
 #include <sys/param.h>
@@ -805,7 +805,7 @@
        netif_tx_request_t txreq;
        struct mbuf *m, *m0 = NULL, *mlast = NULL;
        int receive_pending;
-       RING_IDX req_cons;
+       RING_IDX req_cons, req_prod;
        int queued = 0, m0_len = 0;
        struct xnetback_xstate *xst;
        const bool discard = ((ifp->if_flags & (IFF_UP | IFF_RUNNING)) !=
@@ -813,17 +813,12 @@
 
        XENPRINTF(("xennetback_evthandler "));
        req_cons = xneti->xni_txring.req_cons;
-       while (1) {
-               xen_rmb(); /* be sure to read the request before updating */
-               xneti->xni_txring.req_cons = req_cons;
-               xen_wmb();
-               RING_FINAL_CHECK_FOR_REQUESTS(&xneti->xni_txring,
-                   receive_pending);
-               if (receive_pending == 0)
-                       break;
+again:
+       req_prod = xneti->xni_txring.sring->req_prod;
+       xen_rmb();
+       while (req_cons != req_prod) {
                RING_COPY_REQUEST(&xneti->xni_txring, req_cons,
                    &txreq);
-               xen_rmb();
                XENPRINTF(("%s pkt size %d\n", xneti->xni_if.if_xname,
                    txreq.size));
                req_cons++;
@@ -962,6 +957,12 @@
                        queued = 0;
                }
        }
+       xen_wmb();
+       RING_FINAL_CHECK_FOR_REQUESTS(&xneti->xni_txring, receive_pending);
+       if (receive_pending)
+               goto again;
+       xneti->xni_txring.req_cons = req_cons;
+
        if (m0) {
                /* Queue empty, and still unfinished multi-fragment request */
                printf("%s: dropped unfinished multi-fragment\n",