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tools/lguest: fix missing rmb().

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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>
This commit is contained in:
Rusty Russell 2013-07-02 15:35:13 +09:30
parent 8c6bab4f38
commit 8fd9a6365e
1 changed files with 13 additions and 0 deletions
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13
tools/lguest/lguest.c
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@ -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.