[PATCH v8 09/11] firmware: arm_scmi: Add atomic mode support to virtio transport
Cristian Marussi
cristian.marussi at arm.com
Mon Dec 20 11:56:44 PST 2021
Add support for .mark_txdone and .poll_done transport operations to SCMI
VirtIO transport as pre-requisites to enable atomic operations.
Add a Kernel configuration option to enable SCMI VirtIO transport polling
and atomic mode for selected SCMI transactions while leaving it default
disabled.
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>
---
v7 --> v8
- removed ifdeffery
- reviewed comments
- simplified spinlocking aroung scmi_feed_vq_tx/rx
- added deferred worker for TX replies to aid while polling mode is active
V6 --> V7
- added a few comments about virtio polling internals
- fixed missing list_del on pending_cmds_list processing
- shrinked spinlocked areas in virtio_poll_done
- added proper spinlocking to scmi_vio_complete_cb while scanning list
of pending cmds
---
drivers/firmware/arm_scmi/Kconfig | 15 ++
drivers/firmware/arm_scmi/virtio.c | 291 ++++++++++++++++++++++++++---
2 files changed, 280 insertions(+), 26 deletions(-)
diff --git a/drivers/firmware/arm_scmi/Kconfig b/drivers/firmware/arm_scmi/Kconfig
index d429326433d1..7794bd41eaa0 100644
--- a/drivers/firmware/arm_scmi/Kconfig
+++ b/drivers/firmware/arm_scmi/Kconfig
@@ -118,6 +118,21 @@ config ARM_SCMI_TRANSPORT_VIRTIO_VERSION1_COMPLIANCE
the ones implemented by kvmtool) and let the core Kernel VirtIO layer
take care of the needed conversions, say N.
+config ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
+ bool "Enable atomic mode for SCMI VirtIO transport"
+ depends on ARM_SCMI_TRANSPORT_VIRTIO
+ help
+ Enable support of atomic operation for SCMI VirtIO based transport.
+
+ If you want the SCMI VirtIO based transport to operate in atomic
+ mode, avoiding any kind of sleeping behaviour for selected
+ transactions on the TX path, answer Y.
+
+ Enabling atomic mode operations allows any SCMI driver using this
+ transport to optionally ask for atomic SCMI transactions and operate
+ in atomic context too, at the price of using a number of busy-waiting
+ primitives all over instead. If unsure say N.
+
endif #ARM_SCMI_PROTOCOL
config ARM_SCMI_POWER_DOMAIN
diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
index fd0f6f91fc0b..f589bbcc5db9 100644
--- a/drivers/firmware/arm_scmi/virtio.c
+++ b/drivers/firmware/arm_scmi/virtio.c
@@ -3,8 +3,8 @@
* Virtio Transport driver for Arm System Control and Management Interface
* (SCMI).
*
- * Copyright (C) 2020-2021 OpenSynergy.
- * Copyright (C) 2021 ARM Ltd.
+ * Copyright (C) 2020-2022 OpenSynergy.
+ * Copyright (C) 2021-2022 ARM Ltd.
*/
/**
@@ -38,6 +38,9 @@
* @vqueue: Associated virtqueue
* @cinfo: SCMI Tx or Rx channel
* @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
+ * @deferred_tx_work: Worker for TX deferred replies processing
+ * @deferred_tx_wq: Workqueue for TX deferred replies
+ * @pending_cmds_list: List of pre-fetched commands queueud for later processing
* @is_rx: Whether channel is an Rx channel
* @ready: Whether transport user is ready to hear about channel
* @max_msg: Maximum number of pending messages for this channel.
@@ -49,6 +52,9 @@ struct scmi_vio_channel {
struct virtqueue *vqueue;
struct scmi_chan_info *cinfo;
struct list_head free_list;
+ struct list_head pending_cmds_list;
+ struct work_struct deferred_tx_work;
+ struct workqueue_struct *deferred_tx_wq;
bool is_rx;
bool ready;
unsigned int max_msg;
@@ -65,12 +71,22 @@ struct scmi_vio_channel {
* @input: SDU used for (delayed) responses and notifications
* @list: List which scmi_vio_msg may be part of
* @rx_len: Input SDU size in bytes, once input has been received
+ * @poll_idx: Last used index registered for polling purposes if this message
+ * transaction reply was configured for polling.
+ * Note that since virtqueue used index is an unsigned 16-bit we can
+ * use some out-of-scale values to signify particular conditions.
+ * @poll_lock: Protect access to @poll_idx.
*/
struct scmi_vio_msg {
struct scmi_msg_payld *request;
struct scmi_msg_payld *input;
struct list_head list;
unsigned int rx_len;
+#define VIO_MSG_NOT_POLLED 0xeeeeeeeeUL
+#define VIO_MSG_POLL_DONE 0xffffffffUL
+ unsigned int poll_idx;
+ /* lock to protect access to poll_idx. */
+ spinlock_t poll_lock;
};
/* Only one SCMI VirtIO device can possibly exist */
@@ -81,40 +97,43 @@ static bool scmi_vio_have_vq_rx(struct virtio_device *vdev)
return virtio_has_feature(vdev, VIRTIO_SCMI_F_P2A_CHANNELS);
}
+/* Expect to be called with vioch->lock acquired by the caller and IRQs off */
static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
struct scmi_vio_msg *msg,
struct device *dev)
{
struct scatterlist sg_in;
int rc;
- unsigned long flags;
sg_init_one(&sg_in, msg->input, VIRTIO_SCMI_MAX_PDU_SIZE);
- spin_lock_irqsave(&vioch->lock, flags);
-
rc = virtqueue_add_inbuf(vioch->vqueue, &sg_in, 1, msg, GFP_ATOMIC);
if (rc)
dev_err(dev, "failed to add to RX virtqueue (%d)\n", rc);
else
virtqueue_kick(vioch->vqueue);
- spin_unlock_irqrestore(&vioch->lock, flags);
-
return rc;
}
+/* Expect to be called with vioch->lock acquired by the caller and IRQs off */
+static inline void scmi_vio_feed_vq_tx(struct scmi_vio_channel *vioch,
+ struct scmi_vio_msg *msg)
+{
+ spin_lock(&msg->poll_lock);
+ msg->poll_idx = VIO_MSG_NOT_POLLED;
+ spin_unlock(&msg->poll_lock);
+
+ list_add(&msg->list, &vioch->free_list);
+}
+
static void scmi_finalize_message(struct scmi_vio_channel *vioch,
struct scmi_vio_msg *msg)
{
- if (vioch->is_rx) {
+ if (vioch->is_rx)
scmi_vio_feed_vq_rx(vioch, msg, vioch->cinfo->dev);
- } else {
- /* Here IRQs are assumed to be already disabled by the caller */
- spin_lock(&vioch->lock);
- list_add(&msg->list, &vioch->free_list);
- spin_unlock(&vioch->lock);
- }
+ else
+ scmi_vio_feed_vq_tx(vioch, msg);
}
static void scmi_vio_complete_cb(struct virtqueue *vqueue)
@@ -144,6 +163,7 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
virtqueue_disable_cb(vqueue);
cb_enabled = false;
}
+
msg = virtqueue_get_buf(vqueue, &length);
if (!msg) {
if (virtqueue_enable_cb(vqueue))
@@ -157,7 +177,9 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
scmi_rx_callback(vioch->cinfo,
msg_read_header(msg->input), msg);
+ spin_lock(&vioch->lock);
scmi_finalize_message(vioch, msg);
+ spin_unlock(&vioch->lock);
}
/*
@@ -176,6 +198,34 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
spin_unlock_irqrestore(&vioch->ready_lock, ready_flags);
}
+static void scmi_vio_deferred_tx_worker(struct work_struct *work)
+{
+ unsigned long flags;
+ struct scmi_vio_channel *vioch;
+ struct scmi_vio_msg *msg, *tmp;
+
+ vioch = container_of(work, struct scmi_vio_channel, deferred_tx_work);
+
+ /* Process pre-fetched messages */
+ spin_lock_irqsave(&vioch->lock, flags);
+
+ /* Scan the list of possibly pre-fetched messages during polling. */
+ list_for_each_entry_safe(msg, tmp, &vioch->pending_cmds_list, list) {
+ list_del(&msg->list);
+
+ scmi_rx_callback(vioch->cinfo,
+ msg_read_header(msg->input), msg);
+
+ /* Free the processed message once done */
+ scmi_vio_feed_vq_tx(vioch, msg);
+ }
+
+ spin_unlock_irqrestore(&vioch->lock, flags);
+
+ /* Process possibly still pending messages */
+ scmi_vio_complete_cb(vioch->vqueue);
+}
+
static const char *const scmi_vio_vqueue_names[] = { "tx", "rx" };
static vq_callback_t *scmi_vio_complete_callbacks[] = {
@@ -244,6 +294,19 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
vioch = &((struct scmi_vio_channel *)scmi_vdev->priv)[index];
+ /* Setup a deferred worker for polling. */
+ if (tx && !vioch->deferred_tx_wq) {
+ vioch->deferred_tx_wq =
+ alloc_workqueue(dev_name(&scmi_vdev->dev),
+ WQ_UNBOUND | WQ_FREEZABLE | WQ_SYSFS,
+ 0);
+ if (!vioch->deferred_tx_wq)
+ return -ENOMEM;
+
+ INIT_WORK(&vioch->deferred_tx_work,
+ scmi_vio_deferred_tx_worker);
+ }
+
for (i = 0; i < vioch->max_msg; i++) {
struct scmi_vio_msg *msg;
@@ -257,6 +320,7 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
GFP_KERNEL);
if (!msg->request)
return -ENOMEM;
+ spin_lock_init(&msg->poll_lock);
}
msg->input = devm_kzalloc(cinfo->dev, VIRTIO_SCMI_MAX_PDU_SIZE,
@@ -264,13 +328,12 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
if (!msg->input)
return -ENOMEM;
- if (tx) {
- spin_lock_irqsave(&vioch->lock, flags);
- list_add_tail(&msg->list, &vioch->free_list);
- spin_unlock_irqrestore(&vioch->lock, flags);
- } else {
+ spin_lock_irqsave(&vioch->lock, flags);
+ if (tx)
+ scmi_vio_feed_vq_tx(vioch, msg);
+ else
scmi_vio_feed_vq_rx(vioch, msg, cinfo->dev);
- }
+ spin_unlock_irqrestore(&vioch->lock, flags);
}
spin_lock_irqsave(&vioch->lock, flags);
@@ -296,6 +359,11 @@ static int virtio_chan_free(int id, void *p, void *data)
vioch->ready = false;
spin_unlock_irqrestore(&vioch->ready_lock, flags);
+ if (!vioch->is_rx && vioch->deferred_tx_wq) {
+ destroy_workqueue(vioch->deferred_tx_wq);
+ vioch->deferred_tx_wq = NULL;
+ }
+
scmi_free_channel(cinfo, data, id);
spin_lock_irqsave(&vioch->lock, flags);
@@ -324,7 +392,8 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
}
msg = list_first_entry(&vioch->free_list, typeof(*msg), list);
- list_del(&msg->list);
+ /* Re-init element so we can discern anytime if it is still in-flight */
+ list_del_init(&msg->list);
msg_tx_prepare(msg->request, xfer);
@@ -337,6 +406,19 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,
dev_err(vioch->cinfo->dev,
"failed to add to TX virtqueue (%d)\n", rc);
} else {
+ /*
+ * If polling was requested for this transaction:
+ * - retrieve last used index (will be used as polling reference)
+ * - bind the polled message to the xfer via .priv
+ */
+ if (xfer->hdr.poll_completion) {
+ spin_lock(&msg->poll_lock);
+ msg->poll_idx =
+ virtqueue_enable_cb_prepare(vioch->vqueue);
+ spin_unlock(&msg->poll_lock);
+ /* Ensure initialized msg is visibly bound to xfer */
+ smp_store_mb(xfer->priv, msg);
+ }
virtqueue_kick(vioch->vqueue);
}
@@ -350,10 +432,8 @@ static void virtio_fetch_response(struct scmi_chan_info *cinfo,
{
struct scmi_vio_msg *msg = xfer->priv;
- if (msg) {
+ if (msg)
msg_fetch_response(msg->input, msg->rx_len, xfer);
- xfer->priv = NULL;
- }
}
static void virtio_fetch_notification(struct scmi_chan_info *cinfo,
@@ -361,10 +441,165 @@ static void virtio_fetch_notification(struct scmi_chan_info *cinfo,
{
struct scmi_vio_msg *msg = xfer->priv;
- if (msg) {
+ if (msg)
msg_fetch_notification(msg->input, msg->rx_len, max_len, xfer);
- xfer->priv = NULL;
+}
+
+/**
+ * virtio_mark_txdone - Mark transmission done
+ *
+ * Free only successfully completed polling transfer messages.
+ *
+ * Note that in the SCMI VirtIO transport we never explicitly release timed-out
+ * messages by forcibly re-adding them to the free-list, even on timeout, inside
+ * the TX code path; we instead let IRQ/RX callbacks eventually clean up such
+ * messages once, finally, a late reply is received and discarded (if ever).
+ *
+ * This approach was deemed preferable since those pending timed-out buffers are
+ * still effectively owned by the SCMI platform VirtIO device even after timeout
+ * expiration: forcibly freeing and reusing them before they had been returned
+ * explicitly by the SCMI platform could lead to subtle bugs due to message
+ * corruption.
+ * An SCMI platform VirtIO device which never returns message buffers is
+ * anyway broken and it will quickly lead to exhaustion of available messages.
+ *
+ * For this same reason, here, we take care to free only the successfully
+ * completed polled messages, since they won't be freed elsewhere; late replies
+ * to timed-out polled messages would be anyway freed by RX callbacks instead.
+ *
+ * @cinfo: SCMI channel info
+ * @ret: Transmission return code
+ * @xfer: Transfer descriptor
+ */
+static void virtio_mark_txdone(struct scmi_chan_info *cinfo, int ret,
+ struct scmi_xfer *xfer)
+{
+ unsigned long flags;
+ struct scmi_vio_channel *vioch = cinfo->transport_info;
+ struct scmi_vio_msg *msg = xfer->priv;
+
+ if (!msg)
+ return;
+
+ /* Ensure msg is unbound from xfer before pushing onto the free list */
+ smp_store_mb(xfer->priv, NULL);
+
+ /* Is a successfully completed polled message still to be finalized ? */
+ spin_lock_irqsave(&vioch->lock, flags);
+ if (!ret && xfer->hdr.poll_completion && list_empty(&msg->list))
+ scmi_vio_feed_vq_tx(vioch, msg);
+ spin_unlock_irqrestore(&vioch->lock, flags);
+}
+
+/**
+ * virtio_poll_done - Provide polling support for VirtIO transport
+ *
+ * @cinfo: SCMI channel info
+ * @xfer: Reference to the transfer being poll for.
+ *
+ * VirtIO core provides a polling mechanism based only on last used indexes:
+ * this means that it is possible to poll the virtqueues waiting for something
+ * new to arrive from the host side but the only way to check if the freshly
+ * arrived buffer was what we were waiting for is to compare the newly arrived
+ * message descriptors with the one we are polling on.
+ *
+ * As a consequence it can happen to dequeue something different from the buffer
+ * we were poll-waiting for: if that is the case such early fetched buffers are
+ * then added to a the @pending_cmds_list list for later processing by a
+ * dedicated deferred worker.
+ *
+ * So, basically, once something new is spotted we proceed to de-queue all the
+ * freshly received used buffers until we found the one we were polling on, or,
+ * we have 'seemingly' emptied the virtqueue; if some buffers are still pending
+ * in the vqueue at the end of the polling loop (possible due to inherent races
+ * in virtqueues handling mechanisms), we similarly kick the deferred worker
+ * and let it process those, to avoid indefinitely looping in the .poll_done
+ * helper.
+ *
+ * Note that we do NOT suppress notification with VIRTQ_USED_F_NO_NOTIFY even
+ * when polling since such flag is per-virtqueues and we do not want to
+ * suppress notifications as a whole: so, if the message we are polling for is
+ * delivered via usual IRQs callbacks, on another core which are IRQs-on, it
+ * will be handled as such by scmi_rx_callback() and the polling loop in the
+ * SCMI Core TX path will be transparently terminated anyway.
+ *
+ * Return: True once polling has successfully completed.
+ */
+static bool virtio_poll_done(struct scmi_chan_info *cinfo,
+ struct scmi_xfer *xfer)
+{
+ bool pending, ret = false;
+ unsigned int length, any_prefetched = 0;
+ unsigned long flags;
+ struct scmi_vio_msg *next_msg, *msg = xfer->priv;
+ struct scmi_vio_channel *vioch = cinfo->transport_info;
+
+ if (!msg)
+ return true;
+
+ spin_lock_irqsave(&msg->poll_lock, flags);
+ /* Processed already by other polling loop on another CPU ? */
+ if (msg->poll_idx == VIO_MSG_POLL_DONE) {
+ spin_unlock_irqrestore(&msg->poll_lock, flags);
+ return true;
}
+
+ /* Has cmdq index moved at all ? */
+ pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
+ spin_unlock_irqrestore(&msg->poll_lock, flags);
+ if (!pending)
+ return false;
+
+ spin_lock_irqsave(&vioch->lock, flags);
+ virtqueue_disable_cb(vioch->vqueue);
+
+ /*
+ * If something arrived we cannot be sure, without dequeueing, if it
+ * was the reply to the xfer we are polling for, or, to other, even
+ * possibly non-polling, pending xfers: process all new messages
+ * till the polled-for message is found OR the vqueue is empty.
+ */
+ while ((next_msg = virtqueue_get_buf(vioch->vqueue, &length))) {
+ next_msg->rx_len = length;
+ /* Is the message we were polling for ? */
+ if (next_msg == msg) {
+ ret = true;
+ break;
+ }
+
+ spin_lock(&next_msg->poll_lock);
+ if (next_msg->poll_idx == VIO_MSG_NOT_POLLED) {
+ any_prefetched++;
+ list_add_tail(&next_msg->list,
+ &vioch->pending_cmds_list);
+ } else {
+ next_msg->poll_idx = VIO_MSG_POLL_DONE;
+ }
+ spin_unlock(&next_msg->poll_lock);
+ }
+
+ /*
+ * When the polling loop has successfully terminated if something
+ * else was queued in the meantime, it will be served by a deferred
+ * worker OR by the normal IRQ/callback OR by other poll loops.
+ *
+ * If we are still looking for the polled reply, the polling index has
+ * to be updated to the current vqueue last used index.
+ */
+ if (ret) {
+ pending = !virtqueue_enable_cb(vioch->vqueue);
+ } else {
+ spin_lock(&msg->poll_lock);
+ msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
+ pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
+ spin_unlock(&msg->poll_lock);
+ }
+ spin_unlock_irqrestore(&vioch->lock, flags);
+
+ if (any_prefetched || pending)
+ queue_work(vioch->deferred_tx_wq, &vioch->deferred_tx_work);
+
+ return ret;
}
static const struct scmi_transport_ops scmi_virtio_ops = {
@@ -376,6 +611,8 @@ static const struct scmi_transport_ops scmi_virtio_ops = {
.send_message = virtio_send_message,
.fetch_response = virtio_fetch_response,
.fetch_notification = virtio_fetch_notification,
+ .mark_txdone = virtio_mark_txdone,
+ .poll_done = virtio_poll_done,
};
static int scmi_vio_probe(struct virtio_device *vdev)
@@ -418,6 +655,7 @@ static int scmi_vio_probe(struct virtio_device *vdev)
spin_lock_init(&channels[i].lock);
spin_lock_init(&channels[i].ready_lock);
INIT_LIST_HEAD(&channels[i].free_list);
+ INIT_LIST_HEAD(&channels[i].pending_cmds_list);
channels[i].vqueue = vqs[i];
sz = virtqueue_get_vring_size(channels[i].vqueue);
@@ -506,4 +744,5 @@ const struct scmi_desc scmi_virtio_desc = {
.max_rx_timeout_ms = 60000, /* for non-realtime virtio devices */
.max_msg = 0, /* overridden by virtio_get_max_msg() */
.max_msg_size = VIRTIO_SCMI_MAX_MSG_SIZE,
+ .atomic_enabled = IS_ENABLED(CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE),
};
--
2.17.1
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