[PATCH v2 4/9] firmware: arm_scmi: Add atomic mode support to virtio transport

Cristian Marussi
2022-02-01T17:16:30Z

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@redhat.com>
Cc: Igor Skalkin <igor.skalkin@opensynergy.com>
Cc: Peter Hilber <peter.hilber@opensynergy.com>
Cc: virtualization@lists.linux-foundation.org
Signed-off-by: Cristian Marussi <cristian.marussi@arm.com>
---
v1 --> v2
- shrinked spinlocked section within virtio_poll_done to exclude
virtqueue_poll
- removed poll_lock
- use vio channel refcount acquire/release logic when polling
- using new free_list accessors
- added new dedicated pending_lock to access pending_cmds_list
- fixed a few comments

v0 --> v1
- check for deferred_wq existence before queueing work to avoid
race at driver removal time
- changed mark_txdone decision-logic about message release
- fixed race while checking for msg polled from another thread
- using dedicated poll_status instead of poll_idx upper bits
- pick initial poll_idx earlier inside send_message to avoid missing
early replies
- removed F_NOTIFY mention in comment
- clearing xfer->priv on the IRQ tx path once message has been fetched
- added some store barriers
- updated some comments
---
drivers/firmware/arm_scmi/Kconfig | 15 ++
drivers/firmware/arm_scmi/driver.c | 9 +-
drivers/firmware/arm_scmi/virtio.c | 278 ++++++++++++++++++++++++++++-
3 files changed, 292 insertions(+), 10 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/driver.c b/drivers/firmware/arm_scmi/driver.c
index c2e7897ff56e..dc972a54e93e 100644
--- a/drivers/firmware/arm_scmi/driver.c
+++ b/drivers/firmware/arm_scmi/driver.c
@@ -648,7 +648,8 @@ static void scmi_handle_notification(struct scmi_chan_info *cinfo,

unpack_scmi_header(msg_hdr, &xfer->hdr);
if (priv)
- xfer->priv = priv;
+ /* Ensure order between xfer->priv store and following ops */
+ smp_store_mb(xfer->priv, priv);
info->desc->ops->fetch_notification(cinfo, info->desc->max_msg_size,
xfer);
scmi_notify(cinfo->handle, xfer->hdr.protocol_id,
@@ -680,8 +681,12 @@ static void scmi_handle_response(struct scmi_chan_info *cinfo,
xfer->rx.len = info->desc->max_msg_size;

if (priv)
- xfer->priv = priv;
+ /* Ensure order between xfer->priv store and following ops */
+ smp_store_mb(xfer->priv, priv);
info->desc->ops->fetch_response(cinfo, xfer);
+ if (priv)
+ /* Ensure order between xfer->priv clear and later accesses */
+ smp_store_mb(xfer->priv, NULL);

trace_scmi_rx_done(xfer->transfer_id, xfer->hdr.id,
xfer->hdr.protocol_id, xfer->hdr.seq,
diff --git a/drivers/firmware/arm_scmi/virtio.c b/drivers/firmware/arm_scmi/virtio.c
index 22d5a632262f..46d40da7eeaa 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.
*/

/**
@@ -42,6 +42,10 @@
* @cinfo: SCMI Tx or Rx channel
* @free_lock: Protects access to the @free_list.
* @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_lock: Protects access to the @pending_cmds_list.
+ * @pending_cmds_list: List of pre-fetched commands queueud for later processing
* @is_rx: Whether channel is an Rx channel
* @max_msg: Maximum number of pending messages for this channel.
* @lock: Protects access to all members except users, free_list.
@@ -54,6 +58,11 @@ struct scmi_vio_channel {
/* lock to protect access to the free list. */
spinlock_t free_lock;
struct list_head free_list;
+ /* lock to protect access to the pending list. */
+ spinlock_t pending_lock;
+ struct list_head pending_cmds_list;
+ struct work_struct deferred_tx_work;
+ struct workqueue_struct *deferred_tx_wq;
bool is_rx;
unsigned int max_msg;
/* lock to protect access to all members except users, free_list */
@@ -62,6 +71,12 @@ struct scmi_vio_channel {
refcount_t users;
};

+enum poll_states {
+ VIO_MSG_NOT_POLLED,
+ VIO_MSG_POLLING,
+ VIO_MSG_POLL_DONE,
+};
+
/**
* struct scmi_vio_msg - Transport PDU information
*
@@ -69,12 +84,17 @@ 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.
+ * @poll_status: Polling state for this message.
*/
struct scmi_vio_msg {
struct scmi_msg_payld *request;
struct scmi_msg_payld *input;
struct list_head list;
unsigned int rx_len;
+ unsigned int poll_idx;
+ enum poll_states poll_status;
};

/* Only one SCMI VirtIO device can possibly exist */
@@ -120,6 +140,7 @@ static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
unsigned long flags;
struct device *dev;
DECLARE_COMPLETION_ONSTACK(vioch_shutdown_done);
+ void *deferred_wq = NULL;

/*
* Prepare to wait for the last release if not already released
@@ -130,12 +151,22 @@ static void scmi_vio_channel_cleanup_sync(struct scmi_vio_channel *vioch)
spin_unlock_irqrestore(&vioch->lock, flags);
return;
}
+
vioch->shutdown_done = &vioch_shutdown_done;
vq_name = vioch->is_rx ? "RX" : "TX";
/* vioch->cinfo could be NULLified after the release */
dev = vioch->cinfo->dev;
+
+ if (!vioch->is_rx && vioch->deferred_tx_wq) {
+ deferred_wq = vioch->deferred_tx_wq;
+ /* Cannot be kicked anymore after this...*/
+ vioch->deferred_tx_wq = NULL;
+ }
spin_unlock_irqrestore(&vioch->lock, flags);

+ if (deferred_wq)
+ destroy_workqueue(deferred_wq);
+
scmi_vio_channel_release(vioch);

timeout = msecs_to_jiffies(VIRTIO_MAX_RX_TIMEOUT_MS + 10);
@@ -171,6 +202,8 @@ static void scmi_virtio_put_free_msg(struct scmi_vio_channel *vioch,
{
unsigned long flags;

+ msg->poll_status = VIO_MSG_NOT_POLLED;
+
spin_lock_irqsave(&vioch->free_lock, flags);
list_add_tail(&msg->list, &vioch->free_list);
spin_unlock_irqrestore(&vioch->free_lock, flags);
@@ -241,6 +274,7 @@ 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)) {
@@ -271,6 +305,40 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)
}
}

+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);
+
+ if (!scmi_vio_channel_acquire(vioch))
+ return;
+
+ /* Process pre-fetched messages */
+ spin_lock_irqsave(&vioch->pending_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);
+
+ /* Channel is acquired here and cannot vanish */
+ scmi_rx_callback(vioch->cinfo,
+ msg_read_header(msg->input), msg);
+
+ /* Free the processed message once done */
+ scmi_virtio_put_free_msg(vioch, msg);
+ }
+
+ spin_unlock_irqrestore(&vioch->pending_lock, flags);
+
+ /* Process possibly still pending messages */
+ scmi_vio_complete_cb(vioch->vqueue);
+
+ scmi_vio_channel_release(vioch);
+}
+
static const char *const scmi_vio_vqueue_names[] = { "tx", "rx" };

static vq_callback_t *scmi_vio_complete_callbacks[] = {
@@ -338,6 +406,19 @@ 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;

@@ -405,6 +486,18 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,

spin_lock_irqsave(&vioch->lock, flags);

+ /*
+ * 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) {
+ msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
+ msg->poll_status = VIO_MSG_POLLING;
+ /* Ensure initialized msg is visibly bound to xfer */
+ smp_store_mb(xfer->priv, msg);
+ }
+
rc = virtqueue_add_sgs(vioch->vqueue, sgs, 1, 1, msg, GFP_ATOMIC);
if (rc)
dev_err(vioch->cinfo->dev,
@@ -414,8 +507,11 @@ static int virtio_send_message(struct scmi_chan_info *cinfo,

spin_unlock_irqrestore(&vioch->lock, flags);

- if (rc)
+ if (rc) {
+ /* Ensure order between xfer->priv clear and vq feeding */
+ smp_store_mb(xfer->priv, NULL);
scmi_virtio_put_free_msg(vioch, msg);
+ }

scmi_vio_channel_release(vioch);

@@ -427,10 +523,8 @@ 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,
@@ -438,10 +532,173 @@ 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 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 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 polled messages
+ * that had been somehow replied and not by chance processed on the IRQ path,
+ * since they won't be freed elsewhere; possible late replies to timed-out
+ * polled messages will 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)
+{
+ struct scmi_vio_channel *vioch = cinfo->transport_info;
+ struct scmi_vio_msg *msg = xfer->priv;
+
+ if (!scmi_vio_channel_acquire(vioch))
+ return;
+
+ /* Must be a polled xfer and not already freed on the IRQ path */
+ if (!xfer->hdr.poll_completion || !msg) {
+ scmi_vio_channel_release(vioch);
+ return;
+ }
+
+ /* Ensure msg is unbound from xfer anyway at this point */
+ smp_store_mb(xfer->priv, NULL);
+
+ /* Do not free timedout polled messages */
+ if (ret != -ETIMEDOUT)
+ scmi_virtio_put_free_msg(vioch, msg);
+
+ scmi_vio_channel_release(vioch);
+}
+
+/**
+ * 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 indeed what we were waiting for is to compare the newly
+ * arrived message descriptor 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
+ * busy-waiting helper.
+ *
+ * Note that, since we do NOT have per-message suppress notification mechanism,
+ * the message we are polling for could be alternatively delivered via usual
+ * IRQs callbacks on another core which happened to have IRQs enabled while we
+ * are actively polling for it here: in such a case 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;
+
+ /* Processed already by other polling loop on another CPU ? */
+ if (msg->poll_status == VIO_MSG_POLL_DONE)
+ return true;
+
+ if (!scmi_vio_channel_acquire(vioch))
+ return true;
+
+ /* Has cmdq index moved at all ? */
+ pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
+ if (!pending) {
+ scmi_vio_channel_release(vioch);
+ return false;
+ }
+
+ spin_lock_irqsave(&vioch->lock, flags);
+ virtqueue_disable_cb(vioch->vqueue);
+
+ /*
+ * 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;
+ }
+
+ if (next_msg->poll_status == VIO_MSG_NOT_POLLED) {
+ any_prefetched++;
+
+ spin_lock(&vioch->pending_lock);
+ list_add_tail(&next_msg->list,
+ &vioch->pending_cmds_list);
+ spin_unlock(&vioch->pending_lock);
+ } else {
+ /* We picked another currently polled msg */
+ smp_store_mb(next_msg->poll_status, VIO_MSG_POLL_DONE);
+ }
+ }
+
+ /*
+ * 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 {
+ msg->poll_idx = virtqueue_enable_cb_prepare(vioch->vqueue);
+ pending = virtqueue_poll(vioch->vqueue, msg->poll_idx);
}
+
+ if (vioch->deferred_tx_wq && (any_prefetched || pending))
+ queue_work(vioch->deferred_tx_wq, &vioch->deferred_tx_work);
+
+ spin_unlock_irqrestore(&vioch->lock, flags);
+
+ scmi_vio_channel_release(vioch);
+
+ return ret;
}

static const struct scmi_transport_ops scmi_virtio_ops = {
@@ -453,6 +710,8 @@ 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)
@@ -495,6 +754,8 @@ static int scmi_vio_probe(struct virtio_device *vdev)
spin_lock_init(&channels[i].lock);
spin_lock_init(&channels[i].free_lock);
INIT_LIST_HEAD(&channels[i].free_list);
+ spin_lock_init(&channels[i].pending_lock);
+ INIT_LIST_HEAD(&channels[i].pending_cmds_list);
channels[i].vqueue = vqs[i];

sz = virtqueue_get_vring_size(channels[i].vqueue);
@@ -584,4 +845,5 @@ const struct scmi_desc scmi_virtio_desc = {
.max_rx_timeout_ms = VIRTIO_MAX_RX_TIMEOUT_MS,
.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