Reverse polynomial for an LFSR was initialized improperly which
could generate improper bit sequence in some situations.
This patch implements proper polynomial reversing function.
Fixes: 28ebff11c2dc ("hash: add predictable RSS")
Cc: sta...@dpdk.org
Signed-off-by: Vladimir Medvedkin <vladimir.medved...@intel.com>
---
lib/hash/rte_thash.c | 26 +++++++++++++++++++++++++-
1 file changed, 25 insertions(+), 1 deletion(-)
diff --git a/lib/hash/rte_thash.c b/lib/hash/rte_thash.c
index 10721effe6..99a685f0c8 100644
--- a/lib/hash/rte_thash.c
+++ b/lib/hash/rte_thash.c
@@ -166,6 +166,30 @@ thash_get_rand_poly(uint32_t poly_degree)
RTE_DIM(irreducible_poly_table[poly_degree])];
}
+static inline uint32_t
+get_rev_poly(uint32_t poly, int degree)
+{
+ int i;
+ /*
+ * The implicit highest coefficient of the polynomial
+ * becomes the lowest after reversal.
+ */
+ uint32_t rev_poly = 1;
+ uint32_t mask = (1 << degree) - 1;
+
+ /*
+ * Here we assume "poly" argument is an irreducible polynomial,
+ * thus the lowest coefficient of the "poly" must always be equal to
"1".
+ * After the reversal, this the lowest coefficient becomes the highest
and
+ * it is omitted since the highest coefficient is implicitly determined
by
+ * degree of the polynomial.
+ */
+ for (i = 1; i < degree; i++)
+ rev_poly |= ((poly >> i) & 0x1) << (degree - i);
+
+ return rev_poly & mask;
+}
+
static struct thash_lfsr *
alloc_lfsr(struct rte_thash_ctx *ctx)
{
@@ -185,7 +209,7 @@ alloc_lfsr(struct rte_thash_ctx *ctx)
lfsr->state = rte_rand() & ((1 << lfsr->deg) - 1);
} while (lfsr->state == 0);
/* init reverse order polynomial */
- lfsr->rev_poly = (lfsr->poly >> 1) | (1 << (lfsr->deg - 1));
+ lfsr->rev_poly = get_rev_poly(lfsr->poly, lfsr->deg);
/* init proper rev_state*/
lfsr->rev_state = lfsr->state;
for (i = 0; i <= lfsr->deg; i++)
--
2.34.1
