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    SubjectRe: [RFD] Direct support for the x86 RDRAND instruction
    On 07/29/2011 02:05 PM, Jeff Garzik wrote:
    > This does not cover the one question I [predictably] have: why not do
    > this in rngd, rather than the kernel?

    That is actually why I didn't do the /dev/random aspect of this. I have
    an rngd enabling patch in the works as well.

    The reason for not using RDRAND *only* in rngd is that it is a poor
    match -- RDRAND is designed as a /dev/urandom-type replacement, but it
    is *way* faster than the in-kernel system (since it is all in
    hardware)... plus it is reseeded far more frequently than what is
    possible in software (the architectural spec guarantees a reseed every
    512 reads; I am told by the hardware people that in reality it is way
    more frequent than that.)

    For /dev/random, we do want to be hyper-conservative, though. rngd in
    its current form doesn't deal with fractional entropy, which means
    boiling it down to guaranteed pure entropy; this is an enormous data
    reduction and about three orders of magnitude reduction in bandwidth.
    For /dev/random, I think that is just fine; after all, if you're asking
    for /dev/random, you're asking for security at every cost.

    > Since many (all?) TPM chips include a random number generator, Dell has
    > made sure that most distros have a useful copy of the rng-tools
    > userspace pkg I've been maintaining.
    > It would seem straightforward to add this to rngd, and enable RDRAND on
    > older distros and kernels, as well as current distros / kernels. This
    > also gets useful entropy to /dev/random as part of normal operation,
    > rather than only merely speeding up /dev/urandom.
    > Though for the record, I do agree that this is a nice, small and clean
    > kernel implementation.

    As previously stated, I have that patch in the works as well.


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