virtio: use smp_XX barriers on SMP

#include <linux/virtio_config.h>

#include <linux/device.h>

/* virtio guest is communicating with a virtual "device" that actually runs on

 * a host processor.  Memory barriers are used to control SMP effects. */

#ifdef CONFIG_SMP

/* Where possible, use SMP barriers which are more lightweight than mandatory

 * barriers, because mandatory barriers control MMIO effects on accesses

 * through relaxed memory I/O windows (which virtio does not use). */

#define virtio_mb() smp_mb()

#define virtio_rmb() smp_rmb()

#define virtio_wmb() smp_wmb()

#else

/* We must force memory ordering even if guest is UP since host could be

 * running on another CPU, but SMP barriers are defined to barrier() in that

 * configuration. So fall back to mandatory barriers instead. */

#define virtio_mb() mb()

#define virtio_rmb() rmb()

#define virtio_wmb() wmb()

#endif

#ifdef DEBUG

/* For development, we want to crash whenever the ring is screwed. */

#define BAD_RING(_vq, fmt, args...)				\

	START_USE(vq);

	/* Descriptors and available array need to be set before we expose the

	 * new available array entries. */

	wmb();

	virtio_wmb();

	vq->vring.avail->idx += vq->num_added;

	vq->num_added = 0;

	/* Need to update avail index before checking if we should notify */

	mb();

	virtio_mb();

	if (!(vq->vring.used->flags & VRING_USED_F_NO_NOTIFY))

		/* Prod other side to tell it about changes. */

	}

	/* Only get used array entries after they have been exposed by host. */

	rmb();

	virtio_rmb();

	i = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].id;

	*len = vq->vring.used->ring[vq->last_used_idx%vq->vring.num].len;

	/* We optimistically turn back on interrupts, then check if there was

	 * more to do. */

	vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT;

	mb();

	virtio_mb();

	if (unlikely(more_used(vq))) {

		END_USE(vq);

		return false;