Module Name: src
Committed By: riastradh
Date: Wed Aug 9 08:37:44 UTC 2023
Modified Files:
src/sys/arch/xen/xen: xennetback_xenbus.c
Log Message:
xvif(4): Comment on memory barriers in xennetback_evthandler.
Note which ones appear unnecessary and which ones appear too strong,
but don't change them.
No functional change intended.
To generate a diff of this commit:
cvs rdiff -u -r1.115 -r1.116 src/sys/arch/xen/xen/xennetback_xenbus.c
Please note that diffs are not public domain; they are subject to the
copyright notices on the relevant files.
Modified files:
Index: src/sys/arch/xen/xen/xennetback_xenbus.c
diff -u src/sys/arch/xen/xen/xennetback_xenbus.c:1.115 src/sys/arch/xen/xen/xennetback_xenbus.c:1.116
--- src/sys/arch/xen/xen/xennetback_xenbus.c:1.115 Fri Aug 4 18:41:01 2023
+++ src/sys/arch/xen/xen/xennetback_xenbus.c Wed Aug 9 08:37:44 2023
@@ -1,4 +1,4 @@
-/* $NetBSD: xennetback_xenbus.c,v 1.115 2023/08/04 18:41:01 riastradh Exp $ */
+/* $NetBSD: xennetback_xenbus.c,v 1.116 2023/08/09 08:37:44 riastradh Exp $ */
/*
* Copyright (c) 2006 Manuel Bouyer.
@@ -25,7 +25,7 @@
*/
#include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: xennetback_xenbus.c,v 1.115 2023/08/04 18:41:01 riastradh Exp $");
+__KERNEL_RCSID(0, "$NetBSD: xennetback_xenbus.c,v 1.116 2023/08/09 08:37:44 riastradh Exp $");
#include <sys/types.h>
#include <sys/param.h>
@@ -814,15 +814,28 @@ xennetback_evthandler(void *arg)
XENPRINTF(("xennetback_evthandler "));
req_cons = xneti->xni_txring.req_cons;
while (1) {
+ /*
+ * XXX The xen_rmb here and comment make no sense:
+ * xneti->xni_txring.req_cons is a private variable.
+ */
xen_rmb(); /* be sure to read the request before updating */
xneti->xni_txring.req_cons = req_cons;
+ /* XXX Unclear what this xen_wmb is for. */
xen_wmb();
+ /*
+ * XXX RING_FINAL_CHECK_FOR_REQUESTS issues the most
+ * expensive memory barrier, xen_mb. This should be
+ * used only at the end of the loop after we updating
+ * the producer with the last index of the requests we
+ * consumed in the queue.
+ */
RING_FINAL_CHECK_FOR_REQUESTS(&xneti->xni_txring,
receive_pending);
if (receive_pending == 0)
break;
RING_COPY_REQUEST(&xneti->xni_txring, req_cons,
&txreq);
+ /* XXX Unclear what this xen_rmb is for. */
xen_rmb();
XENPRINTF(("%s pkt size %d\n", xneti->xni_if.if_xname,
txreq.size));
