Re: [PATCH v7 14/16] firmware: arm_scmi: Add atomic mode support to virtio transport

Peter Hilber
2021-12-10T12:26:10Z

On 29.11.21 20:11, Cristian Marussi wrote:
> Add support for .mark_txdone and .poll_done transport operations to SCMI
> VirtIO transport as pre-requisites to enable atomic operations.
>

Hi Cristian,

I see no conceptual problems with this patch. Please find some inline
remarks below.

Best regards,

Peter

> 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>
> ---
> 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 | 241 +++++++++++++++++++++++++++--
> 2 files changed, 243 insertions(+), 13 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..0598e185a786 100644
> --- a/drivers/firmware/arm_scmi/virtio.c
> +++ b/drivers/firmware/arm_scmi/virtio.c
> @@ -38,6 +38,7 @@
> * @vqueue: Associated virtqueue
> * @cinfo: SCMI Tx or Rx channel
> * @free_list: List of unused scmi_vio_msg, maintained for Tx channels only
> + * @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 +50,9 @@ struct scmi_vio_channel {
> struct virtqueue *vqueue;
> struct scmi_chan_info *cinfo;
> struct list_head free_list;
> +#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> + struct list_head pending_cmds_list;
> +#endif
> bool is_rx;
> bool ready;
> unsigned int max_msg;
> @@ -65,12 +69,22 @@ 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 virtqueue used index is an unsigned 16-bit.
> + * @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;
> +#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> +#define VIO_MSG_POLL_DONE 0xffffffffUL

virtqueue_enable_cb_prepare() returns an "opaque unsigned value", so
this special value should not be used for .poll_idx.

> + unsigned int poll_idx;
> + /* lock to protect access to poll_idx. */
> + spinlock_t poll_lock;
> +#endif
> };
>
> /* Only one SCMI VirtIO device can possibly exist */
> @@ -104,17 +118,22 @@ static int scmi_vio_feed_vq_rx(struct scmi_vio_channel *vioch,
> return rc;
> }
>
> +static inline void scmi_vio_feed_vq_tx(struct scmi_vio_channel *vioch,
> + struct scmi_vio_msg *msg)
> +{
> + /* 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);
> +}
> +
> 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)
> @@ -140,6 +159,26 @@ static void scmi_vio_complete_cb(struct virtqueue *vqueue)

Looks like this function could also scmi_rx_callback() for polled
messages, which should probably not happen.

>
> /* IRQs already disabled here no need to irqsave */
> spin_lock(&vioch->lock);
> +#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> + /* At first scan the list of possibly pre-fetched messages */
> + if (!vioch->is_rx) {
> + struct scmi_vio_msg *tmp;
> +
> + list_for_each_entry_safe(msg, tmp,
> + &vioch->pending_cmds_list,
> + list) {
> + list_del(&msg->list);
> + spin_unlock(&vioch->lock);
> +
> + scmi_rx_callback(vioch->cinfo,
> + msg_read_header(msg->input),
> + msg);
> + /* Free the processed message once done */
> + spin_lock(&vioch->lock);
> + list_add(&msg->list, &vioch->free_list);
> + }
> + }
> +#endif
> if (cb_enabled) {
> virtqueue_disable_cb(vqueue);
> cb_enabled = false;
> @@ -257,6 +296,9 @@ static int virtio_chan_setup(struct scmi_chan_info *cinfo, struct device *dev,
> GFP_KERNEL);
> if (!msg->request)
> return -ENOMEM;
> +#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> + spin_lock_init(&msg->poll_lock);
> +#endif
> }
>
> msg->input = devm_kzalloc(cinfo->dev, VIRTIO_SCMI_MAX_PDU_SIZE,
> @@ -324,7 +366,8 @@ 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 +380,20 @@ 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 {
> +#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> + /*
> + * 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);
> + xfer->priv = msg;
> + }
> +#endif
> virtqueue_kick(vioch->vqueue);
> }
>
> @@ -350,10 +407,8 @@ 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,
> @@ -361,11 +416,163 @@ 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;
> +}
> +
> +#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> +/**
> + * 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 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 beeen returned
> + * 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 message exhaustion.
> + *
> + * 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;
> +
> + /* Is a successfully completed polled message still to be finalized ? */
> + spin_lock_irqsave(&msg->poll_lock, flags);

.poll_lock is acquired before vioch->lock (in scmi_vio_feed_vq_tx()),
but e. g. virtio_poll_done() uses the reverse locking order.

> + if (!ret && xfer->hdr.poll_completion && list_empty(&msg->list))
> + scmi_vio_feed_vq_tx(vioch, msg);
> + spin_unlock_irqrestore(&msg->poll_lock, flags);
> +
> + xfer->priv = NULL;
> +}
> +
> +/**
> + * 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 within the
> + * usual VirtIO callbacks; 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 empty the virtqueue.
> + *
> + * 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, it will be handled as such 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 ret;
> + unsigned int poll_idx;
> + unsigned long flags;
> + struct scmi_vio_msg *msg = xfer->priv;
> + struct scmi_vio_channel *vioch = cinfo->transport_info;
> +
> + if (!msg)
> + return true;
> +
> + /*
> + * Keep the spinlocked region as small as possible: don't care if
> + * missing something this time it will be polled again next.
> + */
> + spin_lock_irqsave(&msg->poll_lock, flags);
> + poll_idx = msg->poll_idx;
> + spin_unlock_irqrestore(&msg->poll_lock, flags);
> +
> + /* Processed already by other polling loop on another CPU ? */
> + if (poll_idx == VIO_MSG_POLL_DONE)
> + return true;
> +
> + /* Has cmdq index moved at all ? */
> + ret = virtqueue_poll(vioch->vqueue, poll_idx);

In theory, if polling gets delayed, virtqueue_poll() might run into an
ABA problem, if 2**16 descriptors have been used in the meantime (and
activity stops after this).

I think this could be avoided by clearing .poll_idx of each returned
message everywhere (also in scmi_vio_complete_cb()).

> + if (ret) {
> + struct scmi_vio_msg *next_msg;
> +
> + spin_lock_irqsave(&vioch->lock, flags);
> + virtqueue_disable_cb(vioch->vqueue);
> + /*
> + * If something arrived we cannot be sure if it was the reply to
> + * the xfer we are polling for, or some replies to other, even
> + * possibly non-polling, pending xfers: process all new messages
> + * till the polled-for message is found OR the vqueue is empty.
> + */
> + do {
> + unsigned int length;
> +
> + next_msg = virtqueue_get_buf(vioch->vqueue, &length);
> + if (next_msg) {
> + next_msg->rx_len = length;
> + if (next_msg == msg) {
> + ret = true;
> + break;
> + }
> +
> + list_add_tail(&next_msg->list,
> + &vioch->pending_cmds_list);
> + spin_lock(&next_msg->poll_lock);
> + next_msg->poll_idx = VIO_MSG_POLL_DONE;
> + spin_unlock(&next_msg->poll_lock);
> + ret = false;
> + }
> + } while (next_msg);
> +
> + /*
> + * When the polling loop has successfully terminated simply
> + * restart the vqueue, no matter if something else was queued

"restart the vqueue" should better be phrased as "re-enable the vqueue
callbacks".

> + * in the meantime, it will be served by normal IRQ/callback
> + * or by the next poll loop.
> + *
> + * Update the polling index to the current vqueue last used
> + * index, if still looking for a reply.
> + */
> + if (ret) {
> + virtqueue_enable_cb(vioch->vqueue);

If this function returns false, how is the race described in the
function documentation handled? (The same comment might also apply in
other places.)

> + } else {
> + spin_lock(&msg->poll_lock);
> + msg->poll_idx =
> + virtqueue_enable_cb_prepare(vioch->vqueue);
> + spin_unlock(&msg->poll_lock);
> + }
> + spin_unlock_irqrestore(&vioch->lock, flags);
> }
> +
> + return ret;
> }
> +#endif
>
> static const struct scmi_transport_ops scmi_virtio_ops = {
> .link_supplier = virtio_link_supplier,
> @@ -376,6 +583,10 @@ static const struct scmi_transport_ops scmi_virtio_ops = {
> .send_message = virtio_send_message,
> .fetch_response = virtio_fetch_response,
> .fetch_notification = virtio_fetch_notification,
> +#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> + .mark_txdone = virtio_mark_txdone,
> + .poll_done = virtio_poll_done,
> +#endif
> };
>
> static int scmi_vio_probe(struct virtio_device *vdev)
> @@ -418,6 +629,9 @@ 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);
> +#ifdef CONFIG_ARM_SCMI_TRANSPORT_VIRTIO_ATOMIC_ENABLE
> + INIT_LIST_HEAD(&channels[i].pending_cmds_list);
> +#endif
> channels[i].vqueue = vqs[i];
>
> sz = virtqueue_get_vring_size(channels[i].vqueue);
> @@ -506,4 +720,5 @@ 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),
> };
>