KVM: selftests: Add helpers to probe for NUMA support, and multi-node systems

Add NUMA helpers to probe for support/availability and to check if the
test is running on a multi-node system. The APIs will be used to verify
guest_memfd NUMA support.

Signed-off-by: Shivank Garg <[e-mail]>
[sean: land helpers in numaif.h, add comments, tweak names]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Sean Christopherson <[e-mail]>

tools/testing/selftests/kvm/include/numaif.h

···
 #ifndef SELFTEST_KVM_NUMAIF_H
 #define SELFTEST_KVM_NUMAIF_H
 
+#include <dirent.h>
+
 #include <linux/mempolicy.h>
 
 #include "kvm_syscalls.h"
···
 KVM_SYSCALL_DEFINE(mbind, 6, void *, addr, unsigned long, size, int, mode,
 		   const unsigned long *, nodemask, unsigned long, maxnode,
 		   unsigned int, flags);
+
+static inline int get_max_numa_node(void)
+{
+	struct dirent *de;
+	int max_node = 0;
+	DIR *d;
+
+	/*
+	 * Assume there's a single node if the kernel doesn't support NUMA,
+	 * or if no nodes are found.
+	 */
+	d = opendir("/sys/devices/system/node");
+	if (!d)
+		return 0;
+
+	while ((de = readdir(d)) != NULL) {
+		int node_id;
+		char *endptr;
+
+		if (strncmp(de->d_name, "node", 4) != 0)
+			continue;
+
+		node_id = strtol(de->d_name + 4, &endptr, 10);
+		if (*endptr != '\0')
+			continue;
+
+		if (node_id > max_node)
+			max_node = node_id;
+	}
+	closedir(d);
+
+	return max_node;
+}
+
+static bool is_numa_available(void)
+{
+	/*
+	 * Probe for NUMA by doing a dummy get_mempolicy().  If the syscall
+	 * fails with ENOSYS, then the kernel was built without NUMA support.
+	 * if the syscall fails with EPERM, then the process/user lacks the
+	 * necessary capabilities (CAP_SYS_NICE).
+	 */
+	return !get_mempolicy(NULL, NULL, 0, NULL, 0) ||
+		(errno != ENOSYS && errno != EPERM);
+}
+
+static inline bool is_multi_numa_node_system(void)
+{
+	return is_numa_available() && get_max_numa_node() >= 1;
+}
 
 #endif /* SELFTEST_KVM_NUMAIF_H */