diff options
| -rw-r--r-- | drivers/virtio/virtio_ring.c | 33 | ||||
| -rw-r--r-- | include/linux/virtio_ring.h | 57 |
2 files changed, 63 insertions, 27 deletions
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c index ffd7e7d..245177c 100644 --- a/drivers/virtio/virtio_ring.c +++ b/drivers/virtio/virtio_ring.c @@ -24,27 +24,6 @@ #include <linux/module.h> #include <linux/hrtimer.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-pci does not use). */ -#define virtio_mb(vq) \ - do { if ((vq)->weak_barriers) smp_mb(); else mb(); } while(0) -#define virtio_rmb(vq) \ - do { if ((vq)->weak_barriers) smp_rmb(); else rmb(); } while(0) -#define virtio_wmb(vq) \ - do { if ((vq)->weak_barriers) smp_wmb(); else wmb(); } while(0) -#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(vq) mb() -#define virtio_rmb(vq) rmb() -#define virtio_wmb(vq) wmb() -#endif - #ifdef DEBUG /* For development, we want to crash whenever the ring is screwed. */ #define BAD_RING(_vq, fmt, args...) \ @@ -276,7 +255,7 @@ add_head: /* Descriptors and available array need to be set before we expose the * new available array entries. */ - virtio_wmb(vq); + virtio_wmb(vq->weak_barriers); vq->vring.avail->idx++; vq->num_added++; @@ -312,7 +291,7 @@ bool virtqueue_kick_prepare(struct virtqueue *_vq) START_USE(vq); /* We need to expose available array entries before checking avail * event. */ - virtio_mb(vq); + virtio_mb(vq->weak_barriers); old = vq->vring.avail->idx - vq->num_added; new = vq->vring.avail->idx; @@ -436,7 +415,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len) } /* Only get used array entries after they have been exposed by host. */ - virtio_rmb(vq); + virtio_rmb(vq->weak_barriers); last_used = (vq->last_used_idx & (vq->vring.num - 1)); i = vq->vring.used->ring[last_used].id; @@ -460,7 +439,7 @@ void *virtqueue_get_buf(struct virtqueue *_vq, unsigned int *len) * the read in the next get_buf call. */ if (!(vq->vring.avail->flags & VRING_AVAIL_F_NO_INTERRUPT)) { vring_used_event(&vq->vring) = vq->last_used_idx; - virtio_mb(vq); + virtio_mb(vq->weak_barriers); } #ifdef DEBUG @@ -513,7 +492,7 @@ bool virtqueue_enable_cb(struct virtqueue *_vq) * entry. Always do both to keep code simple. */ vq->vring.avail->flags &= ~VRING_AVAIL_F_NO_INTERRUPT; vring_used_event(&vq->vring) = vq->last_used_idx; - virtio_mb(vq); + virtio_mb(vq->weak_barriers); if (unlikely(more_used(vq))) { END_USE(vq); return false; @@ -553,7 +532,7 @@ bool virtqueue_enable_cb_delayed(struct virtqueue *_vq) /* TODO: tune this threshold */ bufs = (u16)(vq->vring.avail->idx - vq->last_used_idx) * 3 / 4; vring_used_event(&vq->vring) = vq->last_used_idx + bufs; - virtio_mb(vq); + virtio_mb(vq->weak_barriers); if (unlikely((u16)(vq->vring.used->idx - vq->last_used_idx) > bufs)) { END_USE(vq); return false; diff --git a/include/linux/virtio_ring.h b/include/linux/virtio_ring.h index 63c6ea19..ca3ad41 100644 --- a/include/linux/virtio_ring.h +++ b/include/linux/virtio_ring.h @@ -4,6 +4,63 @@ #include <linux/irqreturn.h> #include <uapi/linux/virtio_ring.h> +/* + * Barriers in virtio are tricky. Non-SMP virtio guests can't assume + * they're not on an SMP host system, so they need to assume real + * barriers. Non-SMP virtio hosts could skip the barriers, but does + * anyone care? + * + * For virtio_pci on SMP, we don't need to order with respect to MMIO + * accesses through relaxed memory I/O windows, so smp_mb() et al are + * sufficient. + * + * For using virtio to talk to real devices (eg. other heterogeneous + * CPUs) we do need real barriers. In theory, we could be using both + * kinds of virtio, so it's a runtime decision, and the branch is + * actually quite cheap. + */ + +#ifdef CONFIG_SMP +static inline void virtio_mb(bool weak_barriers) +{ + if (weak_barriers) + smp_mb(); + else + mb(); +} + +static inline void virtio_rmb(bool weak_barriers) +{ + if (weak_barriers) + smp_rmb(); + else + rmb(); +} + +static inline void virtio_wmb(bool weak_barriers) +{ + if (weak_barriers) + smp_wmb(); + else + wmb(); +} +#else +static inline void virtio_mb(bool weak_barriers) +{ + mb(); +} + +static inline void virtio_rmb(bool weak_barriers) +{ + rmb(); +} + +static inline void virtio_wmb(bool weak_barriers) +{ + wmb(); +} +#endif + struct virtio_device; struct virtqueue; |
