Commit 8fd9a636 authored by Rusty Russell's avatar Rusty Russell
Browse files

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: default avatarRusty Russell <rusty@rustcorp.com.au>
parent 8c6bab4f

tools/lguest/lguest.c

+13 −0
Original line number Diff line number Diff line
@@ -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.