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Ha, Intel were *so proud* of their high throughput RDRAND, and now it turns out they leak the values all over the other cores and the microcode patch to fix it... has a 97% performance hit. As in you get ~1/30th of the performance you used to. Whoops! https://www.phoronix.com/scan.php?page=news_item&px=RdRand-3-Percent …
As @sebsued points out, it may well become a DoS vector itself, because if I understand the mitigation right, RDRAND ops are now *serialized against the entire chip memory bus*. They block all other cores from doing any reads and writes that miss cache. https://twitter.com/sebsued/status/1270639456312311809?s=19 …
I just did a test. One thread of RDRAND on an unpatched Xeon E-2144G (8 threads) drops 7-threaded sysbench performance by 10%. The same with the patch, 55%. So with the patch, a rogue core can now halve your memory performance. But it gets better.
Running 8 threads (on an 8-thread E-2144G) of RDRAND in the background, memory throughput drops by *81%* with the patch vs unpatched (91% vs idle). So if someone can spawn a bunch of RDRAND threads, they can drop your memory perf to 19% of what it would be, 9% of nominal.
weren't there some noises on people not trusting rdrand a while back?
Yes, because the NSA might have a backdoor. But it turns out Intel are just incompetent. Who needs backdoors then?
You don't love to see it
Wait so is there any point to rdrand over a software prng now?
You can use both
They've could just "return 4", and that's will be even more fast, why bother
I mean that's basically what AMD did for awhile 
https://arstechnica.com/gadgets/2019/10/how-a-months-old-amd-microcode-bug-destroyed-my-weekend/ …
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