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authorRusty Russell <rusty@rustcorp.com.au>2013-07-02 15:35:13 +0930
committerRusty Russell <rusty@rustcorp.com.au>2013-07-02 15:42:04 +0930
commit8fd9a6365eb8a79a2d8f0e0baa01f4db6345e8dd (patch)
tree35b566aed74e6b4c46cdbff23b2ceda5f4fe5a9b /tools/lguest
parent8c6bab4f3874d31804a00782c48a8f244a0d3cc0 (diff)
tools/lguest: fix missing rmb().
The virtio spec was missing a barrier in example code, so I went back to look at the lguest code. Indeed, we need one. Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Diffstat (limited to 'tools/lguest')
-rw-r--r--tools/lguest/lguest.c13
1 files changed, 13 insertions, 0 deletions
diff --git a/tools/lguest/lguest.c b/tools/lguest/lguest.c
index 07a03452c227..49ed25b5bbb9 100644
--- a/tools/lguest/lguest.c
+++ b/tools/lguest/lguest.c
@@ -177,6 +177,7 @@ static struct termios orig_term;
* in precise order.
*/
#define wmb() __asm__ __volatile__("" : : : "memory")
+#define rmb() __asm__ __volatile__("" : : : "memory")
#define mb() __asm__ __volatile__("" : : : "memory")
/* Wrapper for the last available index. Makes it easier to change. */
@@ -676,6 +677,12 @@ 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 +702,12 @@ 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.
*/

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