Re: use of RDRAND in $random_library
On Fri, 13 Jun 2014, Joey Hess wrote:
> Henrique de Moraes Holschuh wrote:
> > Now, the kernel can soft-blacklist RDRAND (and RDSEED) usage. In that
> > case, the kernel won't use it and it disappears from /proc/cpuinfo, and we
> > could do that also to avoid processor errata, not just due to user request.
> > However, AFAIK kernel blacklisting would not cause the instructions to trap
> > or fail on bare-metal, so userspace could still just use them anyway.
> Not sure what you mean by bare metal here.
Not under a hypervisor.
> > Joey, what does that Haskell lib uses to detect availability of RDRAND?
> int cpu_has_rdrand()
> uint32_t ax,bx,cx,dx,func=1;
> __asm__ volatile ("cpuid":\
> "=a" (ax), "=b" (bx), "=c" (cx), "=d" (dx) : "a" (func));
> return (cx & 0x40000000);
Oh dear. Can you try running that on a kernel that was booted with the
"nordrand" command line parameter? I don't have a box with RDRAND support
to test it myself ATM.
Please try it on bare metal (not on a VM).
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