Subject: Re: [PATCH net-next v2 6/9] devlink: support directly reading from region memory

From: Jiri Pirko <[email-masked]>
Date: Thu, 24 Nov 2022 10:05:39 +0100

Wed, Nov 23, 2022 at 09:38:31PM CET, [email-masked] wrote:
>To read from a region, user space must currently request a new snapshot of
>the region and then read from that snapshot. This can sometimes be overkill
>if user space only reads a tiny portion. They first create the snapshot,
>then request a read, then destroy the snapshot.
>
>For regions which have a single underlying "contents", it makes sense to
>allow supporting direct reading of the region data.
>
>Extend the DEVLINK_CMD_REGION_READ to allow direct reading from a region if
>supported. Instead of reporting a missing snapshot id as invalid, check if
>the region supports direct read and if so switch to the direct access
>method for reading the region data.
>
>Signed-off-by: Jacob Keller <[email-masked]>
>---
>Changes since v1:
>* Update documentation to explain that direct reads are not atomic
>
> .../networking/devlink/devlink-region.rst     | 11 ++++
> include/net/devlink.h                         | 16 +++++
> net/core/devlink.c                            | 66 ++++++++++++++-----
> 3 files changed, 76 insertions(+), 17 deletions(-)
>
>diff --git a/Documentation/networking/devlink/devlink-region.rst b/Documentation/networking/devlink/devlink-region.rst
>index f06dca9a1eb6..6525a9120a4e 100644
>--- a/Documentation/networking/devlink/devlink-region.rst
>+++ b/Documentation/networking/devlink/devlink-region.rst
>@@ -31,6 +31,13 @@ in its ``devlink_region_ops`` structure. If snapshot id is not set in
> the ``DEVLINK_CMD_REGION_NEW`` request kernel will allocate one and send
> the snapshot information to user space.
> 
>+Regions may optionally allow directly reading from their contents without a
>+snapshot. Direct read requests are not atomic. In particular a read request
>+of size 256 bytes or larger will be split into multiple chunks. If atomic
>+access is required, use a snapshot. A driver wishing to enable this for a
>+region should implement the ``.read`` callback in the ``devlink_region_ops``
>+structure.
>+
> example usage
> -------------
> 
>@@ -65,6 +72,10 @@ example usage
>     $ devlink region read pci/0000:00:05.0/fw-health snapshot 1 address 0 length 16
>     0000000000000000 0014 95dc 0014 9514 0035 1670 0034 db30
> 
>+    # Read from the region without a snapshot
>+    $ devlink region read pci/0000:00:05.0/fw-health address 16 length 16
>+    0000000000000010 0000 0000 ffff ff04 0029 8c00 0028 8cc8
>+
> As regions are likely very device or driver specific, no generic regions are
> defined. See the driver-specific documentation files for information on the
> specific regions a driver supports.
>diff --git a/include/net/devlink.h b/include/net/devlink.h
>index 074a79b8933f..02528f736f65 100644
>--- a/include/net/devlink.h
>+++ b/include/net/devlink.h
>@@ -650,6 +650,10 @@ struct devlink_info_req;
>  *            the data variable must be updated to point to the snapshot data.
>  *            The function will be called while the devlink instance lock is
>  *            held.
>+ * @read: callback to directly read a portion of the region. On success,
>+ *        the data pointer will be updated with the contents of the
>+ *        requested portion of the region. The function will be called
>+ *        while the devlink instance lock is held.
>  * @priv: Pointer to driver private data for the region operation
>  */
> struct devlink_region_ops {
>@@ -659,6 +663,10 @@ struct devlink_region_ops {
> 			const struct devlink_region_ops *ops,
> 			struct netlink_ext_ack *extack,
> 			u8 **data);
>+	int (*read)(struct devlink *devlink,
>+		    const struct devlink_region_ops *ops,
>+		    struct netlink_ext_ack *extack,
>+		    u64 offset, u32 size, u8 *data);
> 	void *priv;
> };
> 
>@@ -670,6 +678,10 @@ struct devlink_region_ops {
>  *            the data variable must be updated to point to the snapshot data.
>  *            The function will be called while the devlink instance lock is
>  *            held.
>+ * @read: callback to directly read a portion of the region. On success,
>+ *        the data pointer will be updated with the contents of the
>+ *        requested portion of the region. The function will be called
>+ *        while the devlink instance lock is held.
>  * @priv: Pointer to driver private data for the region operation
>  */
> struct devlink_port_region_ops {
>@@ -679,6 +691,10 @@ struct devlink_port_region_ops {
> 			const struct devlink_port_region_ops *ops,
> 			struct netlink_ext_ack *extack,
> 			u8 **data);
>+	int (*read)(struct devlink_port *port,
>+		    const struct devlink_port_region_ops *ops,
>+		    struct netlink_ext_ack *extack,
>+		    u64 offset, u32 size, u8 *data);
> 	void *priv;
> };
> 
>diff --git a/net/core/devlink.c b/net/core/devlink.c
>index 729e2162a4db..f249b5b57677 100644
>--- a/net/core/devlink.c
>+++ b/net/core/devlink.c
>@@ -6515,6 +6515,26 @@ devlink_region_snapshot_fill(void *cb_priv, u8 *chunk, u32 chunk_size,
> 	return 0;
> }
> 
>+static int
>+devlink_region_port_direct_fill(void *cb_priv, u8 *chunk, u32 chunk_size,
>+				u64 curr_offset, struct netlink_ext_ack *extack)
>+{
>+	struct devlink_region *region = cb_priv;
>+
>+	return region->port_ops->read(region->port, region->port_ops, extack,
>+				      curr_offset, chunk_size, chunk);
>+}
>+
>+static int
>+devlink_region_direct_fill(void *cb_priv, u8 *chunk, u32 chunk_size,
>+			   u64 curr_offset, struct netlink_ext_ack *extack)
>+{
>+	struct devlink_region *region = cb_priv;
>+
>+	return region->ops->read(region->devlink, region->ops, extack,
>+				 curr_offset, chunk_size, chunk);
>+}
>+
> static int devlink_nl_cmd_region_read_dumpit(struct sk_buff *skb,
> 					     struct netlink_callback *cb)
> {
>@@ -6523,12 +6543,12 @@ static int devlink_nl_cmd_region_read_dumpit(struct sk_buff *skb,
> 	u64 ret_offset, start_offset, end_offset = U64_MAX;
> 	struct nlattr **attrs = info->attrs;
> 	struct devlink_port *port = NULL;
>-	struct devlink_snapshot *snapshot;
>+	devlink_chunk_fill_t *region_cb;
> 	struct devlink_region *region;
> 	const char *region_name;
> 	struct devlink *devlink;
> 	unsigned int index;
>-	u32 snapshot_id;
>+	void *region_cb_priv;
> 	void *hdr;
> 	int err;
> 
>@@ -6546,12 +6566,6 @@ static int devlink_nl_cmd_region_read_dumpit(struct sk_buff *skb,
> 		goto out_unlock;
> 	}
> 
>-	if (!attrs[DEVLINK_ATTR_REGION_SNAPSHOT_ID]) {
>-		NL_SET_ERR_MSG(cb->extack, "No snapshot id provided");
>-		err = -EINVAL;
>-		goto out_unlock;
>-	}
>-
> 	if (info->attrs[DEVLINK_ATTR_PORT_INDEX]) {
> 		index = nla_get_u32(info->attrs[DEVLINK_ATTR_PORT_INDEX]);
> 
>@@ -6577,12 +6591,30 @@ static int devlink_nl_cmd_region_read_dumpit(struct sk_buff *skb,
> 	}
> 
> 	snapshot_attr = attrs[DEVLINK_ATTR_REGION_SNAPSHOT_ID];
>-	snapshot_id = nla_get_u32(snapshot_attr);
>-	snapshot = devlink_region_snapshot_get_by_id(region, snapshot_id);
>-	if (!snapshot) {
>-		NL_SET_ERR_MSG_ATTR(cb->extack, snapshot_attr, "requested snapshot does not exist");
>-		err = -EINVAL;
>-		goto out_unlock;
>+	if (!snapshot_attr) {

Hmm. Being pedantic here, it changes userspace expectations:
current - message without snapshotid is errored out
new - message without snapshotid is happily processed

I can imagine some obscure userspace app depending on this behaviour,
in theory.

Safe would be to add new NLA_FLAG attr DEVLINK_ATTR_REGION_DIRECT or
something that would indicate userspace is interested in direct read.
Also, the userspace would know right away it this new functionality
is supported or not by the kernel.

>+		if (!region->ops->read) {
>+			NL_SET_ERR_MSG(cb->extack, "requested region does not support direct read");
>+			err = -EOPNOTSUPP;
>+			goto out_unlock;
>+		}
>+		if (port)
>+			region_cb = &devlink_region_port_direct_fill;
>+		else
>+			region_cb = &devlink_region_direct_fill;
>+		region_cb_priv = region;
>+	} else {
>+		struct devlink_snapshot *snapshot;
>+		u32 snapshot_id;
>+
>+		snapshot_id = nla_get_u32(snapshot_attr);
>+		snapshot = devlink_region_snapshot_get_by_id(region, snapshot_id);
>+		if (!snapshot) {
>+			NL_SET_ERR_MSG_ATTR(cb->extack, snapshot_attr, "requested snapshot does not exist");
>+			err = -EINVAL;
>+			goto out_unlock;
>+		}
>+		region_cb = &devlink_region_snapshot_fill;
>+		region_cb_priv = snapshot;
> 	}
> 
> 	if (attrs[DEVLINK_ATTR_REGION_CHUNK_ADDR] &&
>@@ -6633,9 +6665,9 @@ static int devlink_nl_cmd_region_read_dumpit(struct sk_buff *skb,
> 		goto nla_put_failure;
> 	}
> 
>-	err = devlink_nl_region_read_fill(skb, &devlink_region_snapshot_fill,
>-					  snapshot, start_offset, end_offset,
>-					  &ret_offset, cb->extack);
>+	err = devlink_nl_region_read_fill(skb, region_cb, region_cb_priv,
>+					  start_offset, end_offset, &ret_offset,
>+					  cb->extack);
> 
> 	if (err && err != -EMSGSIZE)
> 		goto nla_put_failure;
>-- 
>2.38.1.420.g319605f8f00e
>