From 8fd9a6365eb8a79a2d8f0e0baa01f4db6345e8dd Mon Sep 17 00:00:00 2001 From: Rusty Russell Date: Tue, 2 Jul 2013 15:35:13 +0930 Subject: [PATCH] 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 --- tools/lguest/lguest.c | 13 +++++++++++++ 1 file changed, 13 insertions(+) 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. @@ -694,6 +701,12 @@ static unsigned wait_for_vq_desc(struct virtqueue *vq, desc = vq->vring.desc; 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.