KVM: selftests: Compute node masks on-demand in xAPIC IPI test

Compute the node masks for the source (from) and destination (to) NUMA
nodes in the xAPIC IPI test instead of pre-filling an array of masks with
node per mask.  Computing the mask on-demand is technically slower, but
doesn't require a large-ish on-stack array, and more importantly allows the
test to use a generic "get next NUMA node" API without having to commit all
of KVM selftests to using a large array of single-bit nodemasks.

Implement said API as a common KVM NUMA API so that it can be used by other
tests, e.g. in guest_memfd tests.  Deliberately make @from "exclusive" as
the anticipated usage in KVM selftests is to select the next, *different*
node, i.e. so that users don't have to copy+paste code to assert that the
found node is different than the starting node.  The obvious downside is
that implementing the exclusive logic forces callers to pass -1 instead of
0 when the goal is to find the first node in the mask, but that arguably
yields more intuitive code anyways.

From an overall test functionality/coverage perspective, no functional
change intended (the walking pattern of node migration should be unchanged).

Reviewed-by: Shivank Garg <shivankg@amd.com>
Link: https://patch.msgid.link/20260903001625.2792367-5-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

1 files changed, 20 insertions, 1 deletions

diff --git a/tools/testing/selftests/kvm/include/numaif.h b/tools/testing/selftests/kvm/include/numaif.h
index 71f261eafc90..299dddff2729 100644
--- a/tools/testing/selftests/kvm/include/numaif.h
+++ b/tools/testing/selftests/kvm/include/numaif.h
@@ -6,7 +6,7 @@
 
 #include <dirent.h>
 
-#include <linux/bitops.h>
+#include <linux/bitmap.h>
 #include <linux/mempolicy.h>
 
 #include "kvm_syscalls.h"
@@ -41,6 +41,25 @@ KVM_SYSCALL_DEFINE(mbind, 6, void *, addr, unsigned long, size, int, mode,
  */
 #define MAXNODE_FOR_MASK(mask) (BITS_PER_TYPE(mask) + 1)
 
+/*
+ * Return the node ID of the next NUMA node in the mask, starting at @from+1.
+ * Guarantees a node is found, and that the found node is not @from.  Pass -1
+ * to find the first node in the mask.
+ */
+static inline int kvm_get_next_numa_node(unsigned long nodemask, int from)
+{
+	const unsigned long nr_bits = BITS_PER_TYPE(nodemask);
+	int to;
+
+	to = find_next_bit(&nodemask, nr_bits, from + 1);
+	if (to == nr_bits)
+		to = find_next_bit(&nodemask, nr_bits, 0);
+
+	TEST_ASSERT(to != nr_bits && to != from,
+		    "Unabled to find second NUMA node (from = %d, to = %d)", from, to);
+	return to;
+}
+
 static inline int get_max_numa_node(void)
 {
 	struct dirent *de;