Re: [libvirt] [PATCH 2/4] xkcd: enable more efficient random number generator

XKCD comic 221 provides a very efficient algorithm for a random number generator, which we can utilize in libvirt. This is enabled simply    LIBVIRT_XKCD=221 /usr/sbin/libvirt The effect can be seen when defining new guests without a MAC address   # virsh define demo.xml   # virsh dumpxml demo | grep mac       <mac address='52:54:00:04:04:04'/> Signed-off-by: Daniel P. Berrange <berrange(a)redhat.com&gt; ---  src/util/virrandom.c | 26 ++++++++++++++++----------  1 file changed, 16 insertions(+), 10 deletions(-) diff --git a/src/util/virrandom.c b/src/util/virrandom.c index 67a8bd0..31d2a8f 100644 --- a/src/util/virrandom.c +++ b/src/util/virrandom.c @@ -34,6 +34,7 @@  #include "virerror.h"  #include "virlog.h"  #include "virstring.h" +#include "virxkcd.h"    #define VIR_FROM_THIS VIR_FROM_NONE   @@ -107,19 +108,24 @@ uint64_t virRandomBits(int nbits)          return 0;      }   -    virMutexLock(&randomLock); +    if (virXKCDIsEnabled(221)) { +        return 4; /* chosen by fair dice roll. +                   * guaranteed to be random. */ +    } else { +        virMutexLock(&randomLock);   -    while (nbits > RANDOM_BITS_PER_ITER) { -        random_r(&randomData, &bits); -        ret = (ret << RANDOM_BITS_PER_ITER) | (bits & RANDOM_BITS_MASK); -        nbits -= RANDOM_BITS_PER_ITER; -    } +        while (nbits > RANDOM_BITS_PER_ITER) { +            random_r(&randomData, &bits); +            ret = (ret << RANDOM_BITS_PER_ITER) | (bits & RANDOM_BITS_MASK); +            nbits -= RANDOM_BITS_PER_ITER; +        }   -    random_r(&randomData, &bits); -    ret = (ret << nbits) | (bits & ((1 << nbits) - 1)); +        random_r(&randomData, &bits); +        ret = (ret << nbits) | (bits & ((1 << nbits) - 1));   -    virMutexUnlock(&randomLock); -    return ret; +        virMutexUnlock(&randomLock); +        return ret; +    }  }