Re: [Qemu-devel] [PATCH v4 23/24] target/ppc: Use qemu_guest_getrandom for DARN

[Laurent Vivier]

On 06/05/2019 19:33, Richard Henderson wrote:
We now have an interface for guest visible random numbers.
Acked-by: David Gibson <da...@gibson.dropbear.id.au>
Signed-off-by: Richard Henderson <richard.hender...@linaro.org>
---
  target/ppc/int_helper.c | 42 +++++++++++++++++++++++++++++------------
  1 file changed, 30 insertions(+), 12 deletions(-)

diff --git a/target/ppc/int_helper.c b/target/ppc/int_helper.c
index f6a088ac08..9059e70b9c 100644
--- a/target/ppc/int_helper.c
+++ b/target/ppc/int_helper.c
@@ -23,6 +23,8 @@
  #include "exec/helper-proto.h"
  #include "crypto/aes.h"
  #include "fpu/softfloat.h"
+#include "qapi/error.h"
+#include "qemu/guest-random.h"
  
  #include "helper_regs.h"
  
/*****************************************************************************/
@@ -158,25 +160,41 @@ uint32_t helper_cmpeqb(target_ulong ra, target_ulong rb)
  #undef hasvalue
  
  /*
- * Return invalid random number.
- *
- * FIXME: Add rng backend or other mechanism to get cryptographically suitable
- * random number
+ * Return a random number.
   */
-target_ulong helper_darn32(void)
+uint64_t helper_darn32(void)
  {
-    return -1;
+    Error *err = NULL;
+    uint32_t ret;
+
+    if (qemu_guest_getrandom(&ret, 4, &err) < 0) {

sizeof(ret)?

+        qemu_log_mask(LOG_UNIMP, "darn: Crypto failure: %s",
+                      error_get_pretty(err));
+        error_free(err);
+        return -1;
+    }
+
+    return ret;
  }
  
-target_ulong helper_darn64(void)
+uint64_t helper_darn64(void)
  {
-    return -1;
+    Error *err = NULL;
+    uint64_t ret;
+
+    do {
+        if (qemu_guest_getrandom(&ret, 8, &err) < 0) {

sizeof(ret)?

+            qemu_log_mask(LOG_UNIMP, "darn: Crypto failure: %s",
+                          error_get_pretty(err));
+            error_free(err);
+            return -1;
+        }
+        /* Since -1 is the error condition, try again for that case.  */
+    } while (unlikely(ret == -1));

I think you don't need to do that, according to documentation, this is 
done by the software:

"Programming Note: When the error value is obtained, software is
expected to repeat the operation. [...]" - PowerISA Version 3.0B

Thanks,
Laurent