libxdp: cache kernel feature probes

headers/xdp/libxdp.h

@@ -44,6 +44,9 @@ typedef int (*libxdp_print_fn_t)(enum libxdp_print_level level,
 
 libxdp_print_fn_t libxdp_set_print(libxdp_print_fn_t fn);
 
+/* Probe and cache kernel feature support during single-threaded startup. */
+void libxdp_cache_kernel_features(void);
+
 
 struct xdp_program;
 struct xdp_multiprog;

lib/libxdp/README.org

@@ -27,6 +27,13 @@ well as reading and writing packets from these sockets.
 Some of the functionality provided by libxdp depends on particular kernel
 features; see the "Kernel feature compatibility" section below for details.
 
+Applications that want to control when libbpf reports unsupported XDP feature
+probes can call =libxdp_cache_kernel_features()= during single-threaded startup.
+This probes and caches XDP frags and device-bound support, so later dispatcher
+attachments reuse the results instead of repeating the probes. Call it with the
+same privileges that will be used for later attachments, and before starting
+threads that use libxdp.
+
 ** Using libxdp from an application
 
 Basic usage of libxdp from an application is quite straight forward. The

lib/libxdp/libxdp.3

@@ -19,6 +19,13 @@ well as reading and writing packets from these sockets.
 .PP
 Some of the functionality provided by libxdp depends on particular kernel
 features; see the "Kernel feature compatibility" section below for details.
+.PP
+Applications that want to control when libbpf reports unsupported XDP feature
+probes can call \fIlibxdp_cache_kernel_features()\fP during single-threaded startup.
+This probes and caches XDP frags and device-bound support, so later dispatcher
+attachments reuse the results instead of repeating the probes. Call it with the
+same privileges that will be used for later attachments, and before starting
+threads that use libxdp.
 .SS "Using libxdp from an application"
 .PP
 Basic usage of libxdp from an application is quite straight forward. The

lib/libxdp/libxdp.c

@@ -232,6 +232,14 @@ static enum {
 	COMPAT_UNSUPPORTED
 } kernel_compat = COMPAT_UNKNOWN;
 
+/* Set by libxdp_cache_kernel_features() during single-threaded startup. */
+static bool kernel_features_cached;
+static bool cached_kernel_frags_support;
+static bool cached_kernel_dev_bound;
+
+static bool kernel_has_frags_support(void);
+static bool kernel_has_dev_bound(void);
+
 static int xdp_multiprog__attach(struct xdp_multiprog *old_mp,
 				 struct xdp_multiprog *mp,
 				 enum xdp_attach_mode mode);
@@ -1664,13 +1672,13 @@ struct xdp_program *xdp_program__clone(struct xdp_program *prog, unsigned int fl
 }
 
 #ifndef HAVE_LIBBPF_BPF_PROGRAM__FLAGS
-static bool kernel_has_frags_support(void)
+static bool probe_kernel_frags_support(void)
 {
 	pr_debug("Can't support frags with old version of libbpf that doesn't support setting program flags.\n");
 	return false;
 }
 
-static bool kernel_has_dev_bound(void)
+static bool probe_kernel_dev_bound(void)
 {
 	pr_debug("Can't bind to device with old version of libbpf that doesn't support setting program flags.\n");
 	return false;
@@ -1723,12 +1731,12 @@ static bool kernel_has_bpf_flag(__u32 flag)
 	return ret;
 }
 
-static bool kernel_has_frags_support(void)
+static bool probe_kernel_frags_support(void)
 {
 	return kernel_has_bpf_flag(BPF_F_XDP_HAS_FRAGS);
 }
 
-static bool kernel_has_dev_bound(void)
+static bool probe_kernel_dev_bound(void)
 {
 	return kernel_has_bpf_flag(BPF_F_XDP_DEV_BOUND_ONLY);
 }
@@ -1763,6 +1771,32 @@ static int xdp_program__set_xdp_dev_bound(struct xdp_program *prog,
 }
 #endif // HAVE_LIBBPF_BPF_PROGRAM__FLAGS
 
+static bool kernel_has_frags_support(void)
+{
+	return kernel_features_cached ? cached_kernel_frags_support :
+		probe_kernel_frags_support();
+}
+
+static bool kernel_has_dev_bound(void)
+{
+	return kernel_features_cached ? cached_kernel_dev_bound :
+		probe_kernel_dev_bound();
+}
+
+/*
+ * Probe kernel features ahead of time so applications can choose when the
+ * libbpf diagnostics from unsupported features are emitted.
+ */
+void libxdp_cache_kernel_features(void)
+{
+	if (kernel_features_cached)
+		return;
+
+	cached_kernel_frags_support = probe_kernel_frags_support();
+	cached_kernel_dev_bound = probe_kernel_dev_bound();
+	kernel_features_cached = true;
+}
+
 static int xdp_program__attach_single(struct xdp_program *prog, int ifindex,
 				      enum xdp_attach_mode mode,
 				      unsigned int flags)

lib/libxdp/libxdp.map

@@ -88,3 +88,7 @@ LIBXDP_1.5.0 {
 LIBXDP_1.6.0 {
 		xdp_multiprog__xdp_dev_bound;
 } LIBXDP_1.5.0;
+
+LIBXDP_1.7.0 {
+		libxdp_cache_kernel_features;
+} LIBXDP_1.6.0;

lib/libxdp/tests/Makefile

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
 
-USER_TARGETS := test_xsk_refcnt test_xdp_frags test_link_detach test_xsk_umem_flags test_xdp_devbound test_xsk_map_leak
+USER_TARGETS := test_xsk_refcnt test_xdp_frags test_link_detach test_xsk_umem_flags test_xdp_devbound test_xsk_map_leak test_kernel_feature_cache
 STATIC_LINK_TARGETS := check_kern_compat test_dispatcher_versions
 BPF_TARGETS := xdp_dispatcher_v1 xdp_dispatcher_v2 xdp_pass
 USER_LIBS := -lpthread

lib/libxdp/tests/test-libxdp.sh

@@ -1,6 +1,6 @@
 # SPDX-License-Identifier: (GPL-2.0 OR BSD-2-Clause)
 
-ALL_TESTS="test_link_so test_link_a test_old_dispatcher test_xdp_devbound test_xdp_frags test_xsk_prog_refcnt_bpffs test_xsk_prog_refcnt_legacy test_xsk_non_privileged test_link_detach test_xsk_umem_flags test_xsk_map_leak"
+ALL_TESTS="test_link_so test_link_a test_kernel_feature_cache test_old_dispatcher test_xdp_devbound test_xdp_frags test_xsk_prog_refcnt_bpffs test_xsk_prog_refcnt_legacy test_xsk_non_privileged test_link_detach test_xsk_umem_flags test_xsk_map_leak"
 
 TESTS_DIR=$(dirname "${BASH_SOURCE[0]}")
 
@@ -21,6 +21,7 @@ test_link_so()
 int main(int argc, char **argv) {
     (void) argc; (void) argv;
     (void) xdp_program__open_file("filename", "section_name", NULL);
+    libxdp_cache_kernel_features();
     return 0;
 }
 EOF
@@ -40,6 +41,7 @@ test_link_a()
 int main(int argc, char **argv) {
     (void) argc; (void) argv;
     (void) xdp_program__open_file("filename", "section_name", NULL);
+    libxdp_cache_kernel_features();
     return 0;
 }
 EOF
@@ -49,6 +51,12 @@ EOF
         return $retval
 }
 
+test_kernel_feature_cache()
+{
+        export LIBXDP_OBJECT_PATH="$TESTS_DIR/.."
+        check_run "$TESTS_DIR/test_kernel_feature_cache"
+}
+
 test_refcnt_once()
 {
 	# We need multiple queues for this test

lib/libxdp/tests/test_kernel_feature_cache.c

@@ -0,0 +1,34 @@
+// SPDX-License-Identifier: (LGPL-2.1 OR BSD-2-Clause)
+#include <xdp/libxdp.h>
+
+static int debug_messages;
+
+static int count_debug(enum libxdp_print_level level, const char *format,
+                       va_list args)
+{
+	(void)format;
+	(void)args;
+
+	if (level == LIBXDP_DEBUG)
+		debug_messages++;
+	return 0;
+}
+
+int main(void)
+{
+	libxdp_print_fn_t old_print;
+	int first_probe_messages;
+
+	old_print = libxdp_set_print(count_debug);
+	libxdp_cache_kernel_features();
+	first_probe_messages = debug_messages;
+	if (!first_probe_messages)
+		return 1;
+
+	libxdp_cache_kernel_features();
+	if (debug_messages != first_probe_messages)
+		return 2;
+
+	libxdp_set_print(old_print);
+	return 0;
+}