netdev: add per-queue statistics

The ethtool-nl family does a good job exposing various protocol
related and IEEE/IETF statistics which used to get dumped under
ethtool -S, with creative names. Queue stats don't have a netlink
API, yet, and remain a lion's share of ethtool -S output for new
drivers. Not only is that bad because the names differ driver to
driver but it's also bug-prone. Intuitively drivers try to report
only the stats for active queues, but querying ethtool stats
involves multiple system calls, and the number of stats is
read separately from the stats themselves. Worse still when user
space asks for values of the stats, it doesn't inform the kernel
how big the buffer is. If number of stats increases in the meantime
kernel will overflow user buffer.

Add a netlink API for dumping queue stats. Queue information is
exposed via the netdev-genl family, so add the stats there.
Support per-queue and sum-for-device dumps. Latter will be useful
when subsequent patches add more interesting common stats than
just bytes and packets.

The API does not currently distinguish between HW and SW stats.
The expectation is that the source of the stats will either not
matter much (good packets) or be obvious (skb alloc errors).

Acked-by: Stanislav Fomichev <sdf@google.com>
Reviewed-by: Amritha Nambiar <amritha.nambiar@intel.com>
Reviewed-by: Xuan Zhuo <xuanzhuo@linux.alibaba.com>
Link: https://lore.kernel.org/r/20240306195509.1502746-2-kuba@kernel.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Documentation/netlink/specs/netdev.yaml

@@ -74,6 +74,10 @@ definitions:
     name: queue-type
     type: enum
     entries: [ rx, tx ]
+  -
+    name: qstats-scope
+    type: flags
+    entries: [ queue ]
 
 attribute-sets:
   -
@@ -265,6 +269,66 @@ attribute-sets:
         doc: ID of the NAPI instance which services this queue.
         type: u32
 
+  -
+    name: qstats
+    doc: |
+      Get device statistics, scoped to a device or a queue.
+      These statistics extend (and partially duplicate) statistics available
+      in struct rtnl_link_stats64.
+      Value of the `scope` attribute determines how statistics are
+      aggregated. When aggregated for the entire device the statistics
+      represent the total number of events since last explicit reset of
+      the device (i.e. not a reconfiguration like changing queue count).
+      When reported per-queue, however, the statistics may not add
+      up to the total number of events, will only be reported for currently
+      active objects, and will likely report the number of events since last
+      reconfiguration.
+    attributes:
+      -
+        name: ifindex
+        doc: ifindex of the netdevice to which stats belong.
+        type: u32
+        checks:
+          min: 1
+      -
+        name: queue-type
+        doc: Queue type as rx, tx, for queue-id.
+        type: u32
+        enum: queue-type
+      -
+        name: queue-id
+        doc: Queue ID, if stats are scoped to a single queue instance.
+        type: u32
+      -
+        name: scope
+        doc: |
+          What object type should be used to iterate over the stats.
+        type: uint
+        enum: qstats-scope
+      -
+        name: rx-packets
+        doc: |
+          Number of wire packets successfully received and passed to the stack.
+          For drivers supporting XDP, XDP is considered the first layer
+          of the stack, so packets consumed by XDP are still counted here.
+        type: uint
+        value: 8 # reserve some attr ids in case we need more metadata later
+      -
+        name: rx-bytes
+        doc: Successfully received bytes, see `rx-packets`.
+        type: uint
+      -
+        name: tx-packets
+        doc: |
+          Number of wire packets successfully sent. Packet is considered to be
+          successfully sent once it is in device memory (usually this means
+          the device has issued a DMA completion for the packet).
+        type: uint
+      -
+        name: tx-bytes
+        doc: Successfully sent bytes, see `tx-packets`.
+        type: uint
+
 operations:
   list:
     -
@@ -405,6 +469,26 @@ operations:
           attributes:
             - ifindex
         reply: *napi-get-op
+    -
+      name: qstats-get
+      doc: |
+        Get / dump fine grained statistics. Which statistics are reported
+        depends on the device and the driver, and whether the driver stores
+        software counters per-queue.
+      attribute-set: qstats
+      dump:
+        request:
+          attributes:
+            - scope
+        reply:
+          attributes:
+            - ifindex
+            - queue-type
+            - queue-id
+            - rx-packets
+            - rx-bytes
+            - tx-packets
+            - tx-bytes
 
 mcast-groups:
   list: