Wed Aug 9 08:38:47 2023 UTC ()

xvif(4): Omit needless membars in xennetback_rx_copy_process.

- No need for barrier around touching req_cons and rsp_prod_pvt,
  which are private.

- RING_PUSH_RESPONSES_AND_CHECK_NOTIFY updates the shared req_prod and
  then issues xen_mb, which is all that we need between the update of
  shared req_prod and hypervisor_notify_via_evtchn.

  (Between updating the shared req_prod and issuing
  hypervisor_notify_via_evtchn, only xen_wmb is needed.  But after
  writing to the shared req_prod, RING_PUSH_REQUESTS_AND_CHECK_NOTIFY
  must also read from the shared rsp_event, which requires the
  store-before-load ordering that only xen_mb provides.)


(riastradh)

diff -r1.121 -r1.122 src/sys/arch/xen/xen/xennetback_xenbus.c

--- src/sys/arch/xen/xen/xennetback_xenbus.c 2023/08/09 08:38:37	1.121
+++ src/sys/arch/xen/xen/xennetback_xenbus.c 2023/08/09 08:38:47	1.122

 @@ -1,41 +1,41 @@
-/*      $NetBSD: xennetback_xenbus.c,v 1.121 2023/08/09 08:38:37 riastradh Exp $      */
+/*      $NetBSD: xennetback_xenbus.c,v 1.122 2023/08/09 08:38:47 riastradh Exp $      */
 /*
  * Copyright (c) 2006 Manuel Bouyer.
+ *
  * Redistribution and use in source and binary forms, with or without
  * modification, are permitted provided that the following conditions
  * are met:
  * 1. Redistributions of source code must retain the above copyright
  *    notice, this list of conditions and the following disclaimer.
  * 2. Redistributions in binary form must reproduce the above copyright
  *    notice, this list of conditions and the following disclaimer in the
  *    documentation and/or other materials provided with the distribution.
+ *
  * THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
  * IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
  * OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
  * IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
  * INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
  * NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
  * DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
  * THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
  * (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
  * THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
  */
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: xennetback_xenbus.c,v 1.121 2023/08/09 08:38:37 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: xennetback_xenbus.c,v 1.122 2023/08/09 08:38:47 riastradh Exp $");
 #include <sys/types.h>
 #include <sys/param.h>
 #include <sys/systm.h>
 #include <sys/kmem.h>
 #include <sys/queue.h>
 #include <sys/kernel.h>
 #include <sys/mbuf.h>
 #include <sys/protosw.h>
 #include <sys/socket.h>
 #include <sys/ioctl.h>
 #include <sys/errno.h>
 #include <sys/device.h>
 @@ -1014,34 +1014,32 @@ xennetback_ifstart(struct ifnet *ifp)
 static void
 xennetback_rx_copy_process(struct ifnet *ifp, struct xnetback_instance *xneti,
 	int queued, int copycnt)
+{
 	int notify;
 	struct xnetback_xstate *xst;
 	if (xennetback_copy(ifp, xneti->xni_gop_copy, copycnt, "Rx") != 0) {
 		/* message already displayed */
 		goto free_mbufs;
+	}
 	/* update pointer */
 	xen_rmb();
 	xneti->xni_rxring.req_cons += queued;
 	xneti->xni_rxring.rsp_prod_pvt += queued;
 	RING_PUSH_RESPONSES_AND_CHECK_NOTIFY(&xneti->xni_rxring, notify);
 	/* send event */
 	if (notify) {
 		xen_rmb();
 		XENPRINTF(("%s receive event\n",
 		    xneti->xni_if.if_xname));
 		hypervisor_notify_via_evtchn(xneti->xni_evtchn);
+	}
 free_mbufs:
 	/* now that data was copied we can free the mbufs */
 	for (int j = 0; j < queued; j++) {
 		xst = &xneti->xni_xstate[j];
 		if (xst->xs_loaded) {
 			bus_dmamap_unload(xneti->xni_xbusd->xbusd_dmat,
 			    xst->xs_dmamap);
 			xst->xs_loaded = false;