find: Add find_first_andnot_bit()

The function helps to implement cpumask_andnot() APIs.

Signed-off-by: Yury Norov [NVIDIA] <yury.norov@gmail.com>
Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: James Morse <james.morse@arm.com>
Reviewed-by: Reinette Chatre <reinette.chatre@intel.com>
Reviewed-by: Fenghua Yu <fenghuay@nvidia.com>
Tested-by: James Morse <james.morse@arm.com>
Tested-by: Tony Luck <tony.luck@intel.com>
Tested-by: Fenghua Yu <fenghuay@nvidia.com>
Link: https://lore.kernel.org/20250515165855.31452-3-james.morse@arm.com

Author: YuryNorov

include/linux/find.h

@@ -29,6 +29,8 @@ unsigned long __find_nth_and_andnot_bit(const unsigned long *addr1, const unsign
 					unsigned long n);
 extern unsigned long _find_first_and_bit(const unsigned long *addr1,
 					 const unsigned long *addr2, unsigned long size);
+unsigned long _find_first_andnot_bit(const unsigned long *addr1, const unsigned long *addr2,
+				 unsigned long size);
 unsigned long _find_first_and_and_bit(const unsigned long *addr1, const unsigned long *addr2,
 				      const unsigned long *addr3, unsigned long size);
 extern unsigned long _find_first_zero_bit(const unsigned long *addr, unsigned long size);
@@ -347,6 +349,29 @@ unsigned long find_first_and_bit(const unsigned long *addr1,
 }
 #endif
 
+/**
+ * find_first_andnot_bit - find the first bit set in 1st memory region and unset in 2nd
+ * @addr1: The first address to base the search on
+ * @addr2: The second address to base the search on
+ * @size: The bitmap size in bits
+ *
+ * Returns the bit number for the first set bit
+ * If no bits are set, returns >= @size.
+ */
+static __always_inline
+unsigned long find_first_andnot_bit(const unsigned long *addr1,
+				 const unsigned long *addr2,
+				 unsigned long size)
+{
+	if (small_const_nbits(size)) {
+		unsigned long val = *addr1 & (~*addr2) & GENMASK(size - 1, 0);
+
+		return val ? __ffs(val) : size;
+	}
+
+	return _find_first_andnot_bit(addr1, addr2, size);
+}
+
 /**
  * find_first_and_and_bit - find the first set bit in 3 memory regions
  * @addr1: The first address to base the search on

lib/find_bit.c

@@ -116,6 +116,17 @@ unsigned long _find_first_and_bit(const unsigned long *addr1,
 EXPORT_SYMBOL(_find_first_and_bit);
 #endif
 
+/*
+ * Find the first bit set in 1st memory region and unset in 2nd.
+ */
+unsigned long _find_first_andnot_bit(const unsigned long *addr1,
+				  const unsigned long *addr2,
+				  unsigned long size)
+{
+	return FIND_FIRST_BIT(addr1[idx] & ~addr2[idx], /* nop */, size);
+}
+EXPORT_SYMBOL(_find_first_andnot_bit);
+
 /*
  * Find the first set bit in three memory regions.
  */