[Valgrind-users] Incorrect results from inline assembly when running under Valgrind

[Jeffrey W. <nol...@gm...>]

I've got some code that determines CPU capabilities for x86
processors. I'm interested in RDRAND and AESNI, and I use three
functions to determine the capabilities: HasIntel(), HasAESNI() and
HasRDRAND().

Debug configurations compiled with GCC and Clang work fine. Release
configuration compiled with GCC and Clang work fine.

However, I get *one* incorrect result when running under Valgrind. The
incorrect result is from HasRDRAND(). Under Valgrind, the result is 0,
which means RDRAND is not available.

I think its related to "Valgrind shows different results for ubuntu
and debian x86 cpuid implementations",
http://sourceforge.net/mailarchive/message.php?msg_id=31873412.

Can anyone confirm this is related to VEX virtual machine? I want to
ensure I'm not seeing success in Debug and Release by dumb-luck (and
possibly have an unforeseen problem to fix).

Thanks in advance.

**********
# Debian 7.3, x64, fully patched
$ uname -a
Linux debian-q500 3.2.0-4-amd64 #1 SMP Debian 3.2.51-1 x86_64 GNU/Linux

# Built from sources a couple of weeks ago
$ valgrind --version
valgrind-3.9.0

**********

static void get_cpuid_info(CPUIDinfo *info, const unsigned int func,
const unsigned int subfunc)
{
    LogDebug("get_cpuid_info");

    UNUSED(info); UNUSED(func); UNUSED(subfunc);

    __asm__ __volatile__ (
        "cpuid"
        : "=a"(info->EAX), "=b"(info->EBX), "=c"(info->ECX), "=d"(info->EDX)
        : "a"(func), "c"(subfunc)
    );
}

int HasIntel()
{
    static int intel_cpu = 0;
    static once_flag flag;

    call_once(flag, []()
    {
        CPUIDinfo info;
        get_cpuid_info(&info,0,0);
        if(memcmp((char *)(&info.EBX), "Genu", 4) == 0 &&
            memcmp((char *)(&info.EDX), "ineI", 4) == 0 &&
            memcmp((char *)(&info.ECX), "ntel", 4) == 0) {
                intel_cpu = 1;
        }
    });

    return intel_cpu;
}

int HasRDRAND()
{
    static volatile int has_rdrand = 0;
    static once_flag flag;

    call_once(flag, []()
    {
        if(HasIntel())
        {
            CPUIDinfo info;
            static const unsigned int RDRAND_CAP_BIT = (1 << 30);
            AC_ASSERT(0x40000000 == RDRAND_CAP_BIT);

            get_cpuid_info(&info,1,0);
            if ((info.ECX & RDRAND_CAP_BIT)==RDRAND_CAP_BIT)
                has_rdrand = 1;
        }
    });

    return has_rdrand;
}

int HasAESNI()
{
    static volatile int has_aesni = 0;
    static once_flag flag;

    call_once(flag, []()
    {
        if(HasIntel())
        {
            CPUIDinfo info;
            static const unsigned int AESNI_CAP_BIT = (1 << 25);
            AC_ASSERT(0x2000000 == AESNI_CAP_BIT);

            get_cpuid_info(&info,1,0);
            if ((info.ECX & AESNI_CAP_BIT)==AESNI_CAP_BIT)
                has_aesni = 1;
        }
    });

    return has_aesni;
}