[PATCH 3/4] virtio_ring: Embed a wrap counter in opaque poll index value
Cristian Marussi
cristian.marussi at arm.com
Tue Feb 8 07:25:19 PST 2022
Exported API virtqueue_poll() can be used to support polling mode operation
on top of virtio layer if needed; currently the parameter last_used_idx is
the opaque value that needs to be passed to the virtqueue_poll() function
to check if there are new pending used buffers in the queue: such opaque
value would have been previously obtained by a call to the API function
virtqueue_enable_cb_prepare().
Since such opaque value is indeed containing simply a snapshot in time of
the internal 16bit last_used_index (roughly), it is possible that, if
another thread calls virtqueue_add_*() at the same time (which existing
drivers don't do, but does not seem to be documented as prohibited
anywhere), and if exactly 2**16 buffers are marked as used between two
successive calls to virtqueue_poll(), the caller is fooled into thinking
that nothing is pending (ABA problem).
Keep a 16bit internal wraps counter per virtqueue and embed it into
the upper 16bits of the returned opaque value, so that the above scenario
can be detected transparently by virtqueue_poll(): this way each single
possible last_used_idx value is really belonging to a different wrap.
The ABA problem is still theoretically possible but this mitigation make
it possibly happen only after 2^32 requests, which seems sufficient in
practice.
Since most drivers usually do not use virtqueue_poll() API in this way to
access the virtqueues in polling mode, make the above behaviour optional
using an internal flag that a driver can set, if desired, calling the new
API virtqueue_use_wrap_counters().
Cc: "Michael S. Tsirkin" <mst at redhat.com>
Cc: Igor Skalkin <igor.skalkin at opensynergy.com>
Cc: Peter Hilber <peter.hilber at opensynergy.com>
Cc: virtualization at lists.linux-foundation.org
Signed-off-by: Cristian Marussi <cristian.marussi at arm.com>
---
---
drivers/virtio/virtio_ring.c | 88 ++++++++++++++++++++++++++++++++++--
include/linux/virtio.h | 2 +
2 files changed, 86 insertions(+), 4 deletions(-)
diff --git a/drivers/virtio/virtio_ring.c b/drivers/virtio/virtio_ring.c
index 962f1477b1fa..00a84b93f4a7 100644
--- a/drivers/virtio/virtio_ring.c
+++ b/drivers/virtio/virtio_ring.c
@@ -12,6 +12,8 @@
#include <linux/hrtimer.h>
#include <linux/dma-mapping.h>
#include <linux/spinlock.h>
+#include <linux/bits.h>
+#include <linux/bitfield.h>
#include <xen/xen.h>
#ifdef DEBUG
@@ -66,6 +68,34 @@
#define LAST_ADD_TIME_INVALID(vq)
#endif
+/*
+ * The provided last_used_idx, as returned by virtqueue_enable_cb_prepare(),
+ * is an opaque value representing the queue state and it is built as follows:
+ *
+ * ---------------------------------------------------------
+ * | vq->wraps | vq->last_used_idx |
+ * 31------------------------------------------------------0
+ *
+ * The MSB 16bits embedding the wraps counter for the underlying virtqueue
+ * is stripped out, using these macros, before reaching into the lower layer
+ * helpers.
+ *
+ * This structure of the opaque value mitigates the scenario in which, when
+ * exactly 2**16 messages are marked as used between two successive calls to
+ * virtqueue_poll(), the caller is fooled into thinking nothing new has arrived
+ * since the pure last_used_idx is exactly the same.
+ */
+#define VRING_IDX_MASK GENMASK(15, 0)
+#define VRING_POLL_GET_IDX(opaque) \
+ ((u16)FIELD_GET(VRING_IDX_MASK, (opaque)))
+
+#define VRING_WRAPS_MASK GENMASK(31, 16)
+#define VRING_POLL_GET_WRAPS(opaque) \
+ ((u16)FIELD_GET(VRING_WRAPS_MASK, (opaque)))
+
+#define VRING_POLL_BUILD_OPAQUE(idx, wraps) \
+ (FIELD_PREP(VRING_WRAPS_MASK, (wraps)) | ((idx) & VRING_IDX_MASK))
+
struct vring_desc_state_split {
void *data; /* Data for callback. */
struct vring_desc *indir_desc; /* Indirect descriptor, if any. */
@@ -114,6 +144,11 @@ struct vring_virtqueue {
/* Last used index we've seen. */
u16 last_used_idx;
+ /* Should we count wraparounds? */
+ bool use_wrap_counter;
+ /* Number of wraparounds */
+ u16 wraps;
+
/* Hint for event idx: already triggered no need to disable. */
bool event_triggered;
@@ -789,6 +824,8 @@ static void *virtqueue_get_buf_ctx_split(struct virtqueue *_vq,
ret = vq->split.desc_state[i].data;
detach_buf_split(vq, i, ctx);
vq->last_used_idx++;
+ if (unlikely(vq->use_wrap_counter))
+ vq->wraps += !vq->last_used_idx;
/* If we expect an interrupt for the next entry, tell host
* by writing event index and flush out the write before
* the read in the next get_buf call. */
@@ -1474,6 +1511,8 @@ static void *virtqueue_get_buf_ctx_packed(struct virtqueue *_vq,
if (unlikely(vq->last_used_idx >= vq->packed.vring.num)) {
vq->last_used_idx -= vq->packed.vring.num;
vq->packed.used_wrap_counter ^= 1;
+ if (unlikely(vq->use_wrap_counter))
+ vq->wraps++;
}
/*
@@ -1709,6 +1748,8 @@ static struct virtqueue *vring_create_virtqueue_packed(
vq->weak_barriers = weak_barriers;
vq->broken = false;
vq->last_used_idx = 0;
+ vq->use_wrap_counter = false;
+ vq->wraps = 0;
vq->event_triggered = false;
vq->num_added = 0;
vq->packed_ring = true;
@@ -2046,16 +2087,49 @@ EXPORT_SYMBOL_GPL(virtqueue_disable_cb);
*/
unsigned virtqueue_enable_cb_prepare(struct virtqueue *_vq)
{
+ unsigned int last_used_idx;
struct vring_virtqueue *vq = to_vvq(_vq);
if (vq->event_triggered)
vq->event_triggered = false;
- return vq->packed_ring ? virtqueue_enable_cb_prepare_packed(_vq) :
- virtqueue_enable_cb_prepare_split(_vq);
+ last_used_idx = vq->packed_ring ?
+ virtqueue_enable_cb_prepare_packed(_vq) :
+ virtqueue_enable_cb_prepare_split(_vq);
+
+ return VRING_POLL_BUILD_OPAQUE(last_used_idx, vq->wraps);
}
EXPORT_SYMBOL_GPL(virtqueue_enable_cb_prepare);
+/**
+ * virtqueue_use_wrap_counter - Enable the virtqueue use_wrap_counter flag
+ * @_vq: the struct virtqueue we're talking about.
+ *
+ * After calling this the core will keep track of virtqueues last_used_idx
+ * in a dedicated wraps counter and such value will be reported embedded in the
+ * MSB 16bit of the opaque value returned by virtqueue_enable_cb_prepare.
+ *
+ * Return: 0 on Success
+ */
+int virtqueue_use_wrap_counter(struct virtqueue *_vq)
+{
+ u8 status;
+ struct vring_virtqueue *vq = to_vvq(_vq);
+
+ if (unlikely(vq->broken))
+ return -EINVAL;
+
+ status = _vq->vdev->config->get_status(_vq->vdev);
+ if (!status || status >= VIRTIO_CONFIG_S_DRIVER_OK)
+ return -EBUSY;
+
+ vq->use_wrap_counter = true;
+ virtio_mb(vq->weak_barriers);
+
+ return 0;
+}
+EXPORT_SYMBOL_GPL(virtqueue_use_wrap_counter);
+
/**
* virtqueue_poll - query pending used buffers
* @_vq: the struct virtqueue we're talking about.
@@ -2072,9 +2146,13 @@ bool virtqueue_poll(struct virtqueue *_vq, unsigned last_used_idx)
if (unlikely(vq->broken))
return false;
+ if (unlikely(vq->wraps != VRING_POLL_GET_WRAPS(last_used_idx)))
+ return true;
+
virtio_mb(vq->weak_barriers);
- return vq->packed_ring ? virtqueue_poll_packed(_vq, last_used_idx) :
- virtqueue_poll_split(_vq, last_used_idx);
+ return vq->packed_ring ?
+ virtqueue_poll_packed(_vq, VRING_POLL_GET_IDX(last_used_idx)) :
+ virtqueue_poll_split(_vq, VRING_POLL_GET_IDX(last_used_idx));
}
EXPORT_SYMBOL_GPL(virtqueue_poll);
@@ -2198,6 +2276,8 @@ struct virtqueue *__vring_new_virtqueue(unsigned int index,
vq->weak_barriers = weak_barriers;
vq->broken = false;
vq->last_used_idx = 0;
+ vq->use_wrap_counter = false;
+ vq->wraps = 0;
vq->event_triggered = false;
vq->num_added = 0;
vq->use_dma_api = vring_use_dma_api(vdev);
diff --git a/include/linux/virtio.h b/include/linux/virtio.h
index 72292a62cd90..3cd428c09f54 100644
--- a/include/linux/virtio.h
+++ b/include/linux/virtio.h
@@ -74,6 +74,8 @@ bool virtqueue_enable_cb(struct virtqueue *vq);
unsigned virtqueue_enable_cb_prepare(struct virtqueue *vq);
+int virtqueue_use_wrap_counter(struct virtqueue *vq);
+
bool virtqueue_poll(struct virtqueue *vq, unsigned);
bool virtqueue_enable_cb_delayed(struct virtqueue *vq);
--
2.17.1
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