2 files changed, 42 insertions, 13 deletions
diff --git a/src/lib/entropy/p9_darn/info.txt b/src/lib/entropy/p9_darn/info.txt
index 30f76603b..47d99aa3d 100644
--- a/src/lib/entropy/p9_darn/info.txt
+++ b/src/lib/entropy/p9_darn/info.txt
@@ -6,14 +6,6 @@ ENTROPY_SRC_DARN -> 20180610
 p9_darn.h
 </header:internal>
 
-<cc>
-gcc:7.0
-</cc>
-
-<isa>
-power9
-</isa>
-
 <arch>
 ppc64
 </arch>
diff --git a/src/lib/entropy/p9_darn/p9_darn.cpp b/src/lib/entropy/p9_darn/p9_darn.cpp
index aa88b8301..8731b94e1 100644
--- a/src/lib/entropy/p9_darn/p9_darn.cpp
+++ b/src/lib/entropy/p9_darn/p9_darn.cpp
@@ -9,19 +9,56 @@
 
 namespace Botan {
 
+namespace {
+
+bool read_darn(secure_vector<uint64_t>& seed)
+   {
+   const size_t DARN_RETRIES = 512;
+
+   for(size_t i = 0; i != DARN_RETRIES; ++i)
+      {
+      uint64_t r = 0;
+
+      // DARN 0: 32-bit conditioned, 1: 64-bit condition, 2: 64-bit raw (ala RDSEED)
+      asm volatile("darn %0, 2" : "=r" (r));
+
+      // DARN indicates error by 0xFF..FF, ie is biased (!?!?)
+      if((~r) != 0)
+         {
+         seed.push_back(r);
+         return true;
+         }
+      }
+
+   return false; // failed to produce an output after many attempts
+   }
+
+}
+
 size_t POWER9_DARN::poll(RandomNumberGenerator& rng)
    {
+   const size_t DARN_BYTES = 1024;
+   static_assert(DARN_BYTES % 8 == 0, "Bad DARN configuration");
+
    if(CPUID::has_darn_rng())
       {
-      secure_vector<uint64_t> seed(64);
+      secure_vector<uint64_t> seed;
+      seed.reserve(DARN_BYTES / 8);
 
-      for(size_t i = 0; i != seed.size(); ++i)
-         seed[i] = __builtin_darn_raw();
+      for(size_t p = 0; p != DARN_BYTES / 8; ++p)
+         {
+         if(!read_darn(seed))
+            break;
+         }
 
-      rng.add_entropy(reinterpret_cast<const uint8_t*>(seed.data()),
-                      seed.size() * sizeof(uint64_t));
+      if(seed.size() > 0)
+         {
+         rng.add_entropy(reinterpret_cast<const uint8_t*>(seed.data()),
+                         seed.size() * sizeof(uint32_t));
+         }
       }
 
+   // DARN is used but not trusted
    return 0;
    }