kselftest/arm64: Skip non-power of 2 SVE vector lengths in fp-stress

As documented in issue C215 in the known issues list for DDI0487I.a [1] Arm
will be making a retroactive change to SVE to remove the possibility of
selecting non power of two vector lengths. This has no impact on existing
physical implementations but most virtual implementations have implemented
the full range of permissible vector lengths. Given how demanding fp-stress
is for these implementations update to only attempt to enumerate the power
of two vector lengths, reducing the load created on existing virtual
implementations and only exercising the functionality that will be seen in
physical implementations.

[1] https://developer.arm.com/documentation/102105/ia-00/Signed-off-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20221220-arm64-fp-stress-pow2-v1-1-d0ce756b57af@kernel.orgSigned-off-by: Catalin Marinas <catalin.marinas@arm.com>

tools/testing/selftests/arm64/fp/fp-stress.c

@@ -377,7 +377,7 @@ static void probe_vls(int vls[], int *vl_count, int set_vl)
 
 	*vl_count = 0;
 
-	for (vq = SVE_VQ_MAX; vq > 0; --vq) {
+	for (vq = SVE_VQ_MAX; vq > 0; vq /= 2) {
 		vl = prctl(set_vl, vq * 16);
 		if (vl == -1)
 			ksft_exit_fail_msg("SET_VL failed: %s (%d)\n",
@@ -385,6 +385,9 @@ static void probe_vls(int vls[], int *vl_count, int set_vl)
 
 		vl &= PR_SVE_VL_LEN_MASK;
 
+		if (*vl_count && (vl == vls[*vl_count - 1]))
+			break;
+
 		vq = sve_vq_from_vl(vl);
 
 		vls[*vl_count] = vl;