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Date: Tue, 19 Feb 2013 08:59:28 +1030
From: Rusty Russell <rusty@...tcorp.com.au>
To: virtualization@...ts.linux-foundation.org
Cc: sjur.brandeland@...ricsson.com, mst@...hat.com,
linux-kernel@...r.kernel.org, Rusty Russell <rusty@...tcorp.com.au>
Subject: [PATCH 1/5] virtio_ring: expose virtio barriers for use in vringh.
The host side of ring needs this logic too.
Signed-off-by: Rusty Russell <rusty@...tcorp.com.au>
---
drivers/virtio/virtio_ring.c | 33 +++++-------------------
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 63c6ea1..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;
--
1.7.10.4
--
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