[src/trunk]: src/sys/arch/x86/x86 x86: Move lfence into _bus_dmamap_sync and ...

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

details:   https://anonhg.NetBSD.org/src/rev/52935f6cd238
branches:  trunk
changeset: 368462:52935f6cd238
user:      riastradh <riastradh%NetBSD.org@localhost>
date:      Wed Jul 13 00:12:20 2022 +0000

description:
x86: Move lfence into _bus_dmamap_sync and comment it.

No functional change intended.  This just keeps the bus_dma_* and
_bus_dma_* functions organized more consistently.

diffstat:

 sys/arch/x86/x86/bus_dma.c |  17 ++++++++++++-----
 1 files changed, 12 insertions(+), 5 deletions(-)

diffs (45 lines):

diff -r 5881000febea -r 52935f6cd238 sys/arch/x86/x86/bus_dma.c
--- a/sys/arch/x86/x86/bus_dma.c        Tue Jul 12 23:47:00 2022 +0000
+++ b/sys/arch/x86/x86/bus_dma.c        Wed Jul 13 00:12:20 2022 +0000
@@ -1,4 +1,4 @@
-/*     $NetBSD: bus_dma.c,v 1.84 2022/01/22 15:10:32 skrll Exp $       */
+/*     $NetBSD: bus_dma.c,v 1.85 2022/07/13 00:12:20 riastradh Exp $   */
 
 /*-
  * Copyright (c) 1996, 1997, 1998, 2007, 2020 The NetBSD Foundation, Inc.
@@ -31,7 +31,7 @@
  */
 
 #include <sys/cdefs.h>
-__KERNEL_RCSID(0, "$NetBSD: bus_dma.c,v 1.84 2022/01/22 15:10:32 skrll Exp $");
+__KERNEL_RCSID(0, "$NetBSD: bus_dma.c,v 1.85 2022/07/13 00:12:20 riastradh Exp $");
 
 /*
  * The following is included because _bus_dma_uiomove is derived from
@@ -816,6 +816,16 @@
 #endif
 
        /*
+        * The caller has been alerted to DMA completion by reading a
+        * register or DMA descriptor, and is about to read out of the
+        * DMA memory buffer that the device filled.  LFENCE ensures
+        * that these happen in order, so that the caller doesn't
+        * proceed to read any stale data from cache or speculation.
+        */
+       if (ops & BUS_DMASYNC_POSTREAD)
+               x86_lfence();
+
+       /*
         * If we're not bouncing, just return; nothing to do.
         */
        if (len == 0 || cookie == NULL ||
@@ -1340,9 +1350,6 @@
                return;
        }
 
-       if (ops & BUS_DMASYNC_POSTREAD)
-               x86_lfence();
-
        _bus_dmamap_sync(t, p, o, l, ops);
 }