1 files changed, 23 insertions, 9 deletions

diff --git a/tools/lguest/lguest.c b/tools/lguest/lguest.c
index 07a03452c227..68f67cf3d318 100644
--- a/tools/lguest/lguest.c
+++ b/tools/lguest/lguest.c
@@ -42,14 +42,6 @@
 #include <pwd.h>
 #include <grp.h>
 
-#include <linux/virtio_config.h>
-#include <linux/virtio_net.h>
-#include <linux/virtio_blk.h>
-#include <linux/virtio_console.h>
-#include <linux/virtio_rng.h>
-#include <linux/virtio_ring.h>
-#include <asm/bootparam.h>
-#include "../../include/linux/lguest_launcher.h"
 /*L:110
  * We can ignore the 43 include files we need for this program, but I do want
  * to draw attention to the use of kernel-style types.
@@ -65,6 +57,15 @@ typedef uint16_t u16;
 typedef uint8_t u8;
 /*:*/
 
+#include <linux/virtio_config.h>
+#include <linux/virtio_net.h>
+#include <linux/virtio_blk.h>
+#include <linux/virtio_console.h>
+#include <linux/virtio_rng.h>
+#include <linux/virtio_ring.h>
+#include <asm/bootparam.h>
+#include "../../include/linux/lguest_launcher.h"
+
 #define BRIDGE_PFX "bridge:"
 #ifndef SIOCBRADDIF
 #define SIOCBRADDIF     0x89a2          /* add interface to bridge      */
@@ -177,7 +178,8 @@ static struct termios orig_term;
  * in precise order.
  */
 #define wmb() __asm__ __volatile__("" : : : "memory")
-#define mb() __asm__ __volatile__("" : : : "memory")
+#define rmb() __asm__ __volatile__("lock; addl $0,0(%%esp)" : : : "memory")
+#define mb() __asm__ __volatile__("lock; addl $0,0(%%esp)" : : : "memory")
 
 /* Wrapper for the last available index.  Makes it easier to change. */
 #define lg_last_avail(vq)       ((vq)->last_avail_idx)
@@ -676,6 +678,12 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
                 errx(1, "Guest moved used index from %u to %u",
                      last_avail, vq->vring.avail->idx);
 
+        /* 
+         * Make sure we read the descriptor number *after* we read the ring
+         * update; don't let the cpu or compiler change the order.
+         */
+        rmb();
+
         /*
          * Grab the next descriptor number they're advertising, and increment
          * the index we've seen.
@@ -695,6 +703,12 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq,
         i = head;
 
         /*
+         * We have to read the descriptor after we read the descriptor number,
+         * but there's a data dependency there so the CPU shouldn't reorder
+         * that: no rmb() required.
+         */
+
+        /*
          * If this is an indirect entry, then this buffer contains a descriptor
          * table which we handle as if it's any normal descriptor chain.
          */