Re: [PATCH net-next 03/14] net: rtnetlink: RTM_GETSTATS: Allow filtering inside nests

On Thu, 24 Feb 2022 15:33:24 +0200 Ido Schimmel wrote:

From: Petr Machata <petrm@nvidia.com>

The filter_mask field of RTM_GETSTATS header determines which top-level
attributes should be included in the netlink response. This saves
processing time by only including the bits that the user cares about
instead of always dumping everything. This is doubly important for
HW-backed statistics that would typically require a trip to the device to
fetch the stats.

So far there was only one HW-backed stat suite per attribute. However,
IFLA_STATS_LINK_OFFLOAD_XSTATS is a nest, and will gain a new stat suite in
the following patches. It would therefore be advantageous to be able to
filter within that nest, and select just one or the other HW-backed
statistics suite.

Extend rtnetlink so that RTM_GETSTATS permits attributes in the payload.
The scheme is as follows:

    - RTM_GETSTATS
	- struct if_stats_msg
	- attr nest IFLA_STATS_GET_FILTERS
	    - attr IFLA_STATS_LINK_OFFLOAD_XSTATS
		- struct nla_bitfield32 filter_mask

This scheme reuses the existing enumerators by nesting them in a dedicated
context attribute. This is covered by policies as usual, therefore a
gradual opt-in is possible. Currently only IFLA_STATS_LINK_OFFLOAD_XSTATS
nest has filtering enabled, because for the SW counters the issue does not
seem to be that important.

rtnl_offload_xstats_get_size() and _fill() are extended to observe the
requested filters.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>

I don't think we need the .strict_start_type if the policy is not used
in parse calls with a _deprecated() suffix, no?

+	[IFLA_STATS_GET_FILTERS] = { .type = NLA_NESTED },

NLA_POLICY_NESTED()? Maybe one day we'll have policy dumping 
for rtnetlink and it'll be useful to have policies linked up.

+};
+
+#define RTNL_STATS_OFFLOAD_XSTATS_VALID ((1 << __IFLA_OFFLOAD_XSTATS_MAX) - 1)
+
+static const struct nla_policy
+rtnl_stats_get_policy_filters[IFLA_STATS_MAX + 1] = {
+	[IFLA_STATS_UNSPEC] = { .strict_start_type = 1 },
+	[IFLA_STATS_LINK_OFFLOAD_XSTATS] =
+			NLA_POLICY_BITFIELD32(RTNL_STATS_OFFLOAD_XSTATS_VALID),
+};
+
+static int rtnl_stats_get_parse_filters(struct nlattr *ifla_filters,
+					struct rtnl_stats_dump_filters *filters,
+					struct netlink_ext_ack *extack)
+{
+	struct nlattr *tb[IFLA_STATS_MAX + 1];
+	int err;
+	int at;
+
+	err = nla_parse_nested(tb, IFLA_STATS_MAX, ifla_filters,
+			       rtnl_stats_get_policy_filters, extack);
+	if (err < 0)
+		return err;
+
+	for (at = 1; at <= IFLA_STATS_MAX; at++) {
+		if (tb[at]) {
+			if (!(filters->mask[0] & IFLA_STATS_FILTER_BIT(at))) {
+				NL_SET_ERR_MSG(extack, "Filtered attribute not enabled in filter_mask");
+				return -EINVAL;
+			}
+			filters->mask[at] = nla_get_bitfield32(tb[at]).value;

Why use bitfield if we only use the .value, a u32 would do?

+		}
+	}