On Sunday, October 18, 2015 at 8:23:32 PM UTC-4, Jeffrey Walton wrote:
>
>
> The only thing left to do with it is
>> a) verify that I actually did the opcode right [1]
>>
>
> By the way, I think RDSEED detection code should look similar to the
> following. Also see the discussion of Highest Function level at
> https://en.wikipedia.org/wiki/CPUID#EAX.3D0:_Highest_Function_Parameter
> ...
>
Here's what the RDSEED detection code ended up being. AMD does not offer
RDSEED, so we need to filter the call for Intel processors.
static bool IsIntel(const word32 output[4])
{
// This is the "GenuineIntel" string
return (output[1] /*EBX*/ == 0x756e6547) &&
(output[2] /*ECX*/ == 0x6c65746e) &&
(output[3] /*EDX*/ == 0x49656e69);
}
static bool IsAMD(const word32 output[4])
{
// This is the "AuthenticAMD" string
return (output[1] /*EBX*/ == 0x68747541) &&
(output[2] /*ECX*/ == 0x69746E65) &&
(output[3] /*EDX*/ == 0x444D4163);
}
static bool RDSEED_Runtime_Helper()
{
#if defined(CRYPTOPP_CPUID_AVAILABLE)
bool rdseed = false; word32 output[4];
if (CpuId(0, output))
{
// Only Intel supports RDSEED at the moment.
if (IsIntel(output))
{
if (output[0] /*EAX*/ >= 7 && CpuId(7, output))
{
static const unsigned int RDSEED_FLAG = (1 << 18);
rdseed = !!(output[1] /*EBX*/ & RDSEED_FLAG);
}
}
}
return rdseed;
#else
return false;
#endif
}
And the updated RDRAND helper routine:
static bool RDRAND_Runtime_Helper()
{
#if defined(CRYPTOPP_CPUID_AVAILABLE)
bool rdrand = false; word32 output[4];
if (CpuId(0, output))
{
if (IsIntel(output) || IsAMD(output))
{
if (output[0] /*EAX*/ >= 1 && CpuId(1, output))
{
static const unsigned int RDRAND_FLAG = (1 << 30);
rdrand = !!(output[2] /*ECX*/ & RDRAND_FLAG);
}
}
}
return rdrand;
#else
return false;
#endif
}
Jeff
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