linux-kernel - RE: [PATCH 1/2] x86/random: Retry on RDSEED failure

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Date: Wed, 14 Feb 2024 15:18:00 +0000
From: "Reshetova, Elena" <elena.reshetova@...el.com>
To: "Reshetova, Elena" <elena.reshetova@...el.com>, "Jason A. Donenfeld"
	<Jason@...c4.com>, Theodore Ts'o <tytso@....edu>, Dave Hansen
	<dave.hansen@...ux.intel.com>
CC: "Kirill A. Shutemov" <kirill.shutemov@...ux.intel.com>, Thomas Gleixner
	<tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>, Borislav Petkov
	<bp@...en8.de>, "H. Peter Anvin" <hpa@...or.com>, "x86@...nel.org"
	<x86@...nel.org>, Kuppuswamy Sathyanarayanan
	<sathyanarayanan.kuppuswamy@...ux.intel.com>, "Nakajima, Jun"
	<jun.nakajima@...el.com>, Tom Lendacky <thomas.lendacky@....com>, "Kalra,
 Ashish" <ashish.kalra@....com>, Sean Christopherson <seanjc@...gle.com>,
	"linux-coco@...ts.linux.dev" <linux-coco@...ts.linux.dev>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: RE: [PATCH 1/2] x86/random: Retry on RDSEED failure

> This is a great summary of options, thank you Jason!
> My proposal would be to wait on result of our internal investigation
> before proceeding to choose the approach.

Hi everyone, 

I am finally able to share the result of my AR and here is the statement
about rdrand/rdseed on Intel platforms:

"The RdRand in a non-defective device is designed to be faster than the bus,
so when a core accesses the output from the DRNG, it will always get a
random number.
As a result, it is hard to envision a scenario where the RdRand, on a fully
functional device, will underflow.
The carry flag after RdRand signals an underflow so in the case of a defective chip,
this will prevent the code thinking it has a random number when it does not.

RdSeed however is limited by the speed of the noise source. So it is not faster
than the bus and there may be an underflow signaled by the carry flag. 
When reading for multiple values, the total throughput of RdSeed random
numbers varies over different products due to variation in the silicon processes,
operating voltage and speed vs power tradeoffs.
The throughput is shared between the cores"

In addition there is a plan to publish a whitepaper and add clarifications to
Intel official documentation on this topic, but this would obviously take longer. 

Best Regards,
Elena.