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From: "Jason A. Donenfeld" <Jason@zx2c4.com>
Date: Wed, 14 Feb 2024 21:04:34 +0100
Message-ID: <CAHmME9pzOTdkNr=mM7yKKqLWApQ5cxjvb7R9C2eQ2QFeUEqT6A@mail.gmail.com>
Subject: Re: [PATCH 1/2] x86/random: Retry on RDSEED failure
To: Tom Lendacky <thomas.lendacky@amd.com>, "Reshetova, Elena" <elena.reshetova@intel.com>, 
	Borislav Petkov <bp@alien8.de>
Cc: "Theodore Ts'o" <tytso@mit.edu>, Dave Hansen <dave.hansen@linux.intel.com>, 
	"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com>, Thomas Gleixner <tglx@linutronix.de>, 
	Ingo Molnar <mingo@redhat.com>, "H. Peter Anvin" <hpa@zytor.com>, "x86@kernel.org" <x86@kernel.org>, 
	Kuppuswamy Sathyanarayanan <sathyanarayanan.kuppuswamy@linux.intel.com>, 
	"Nakajima, Jun" <jun.nakajima@intel.com>, "Kalra, Ashish" <ashish.kalra@amd.com>, 
	Sean Christopherson <seanjc@google.com>, 
	"linux-coco@lists.linux.dev" <linux-coco@lists.linux.dev>, 
	"linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>
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Hi Tom,

On Wed, Feb 14, 2024 at 8:46=E2=80=AFPM Tom Lendacky <thomas.lendacky@amd.c=
om> wrote:
> Don't forget that Linux will run on older hardware as well, so the 10
> retries might be valid for that. Or do you intend this change purely for =
CVMs?

Oh, grr, darnit. That is indeed a very important detail. I meant this
for generic code, so yea, if it's actually just Zen3+, then this won't
fly.

AMD people, Intel people: what are the fullest statements we can rely
on here? Do the following two statements work?

1) On newer chips, RDRAND never fails.
2) On older chips, RDRAND never fails if you try 10 times in a loop,
unless you consider host->guest attacks, which we're not, because CoCo
is only a thing on the newer chips.

If those hold true, then the course of action would be to just add a
WARN_ON(!ok) but keep the loop as-is.

(Anyway, I posted
https://lore.kernel.org/lkml/20240214195744.8332-1-Jason@zx2c4.com/
just before seeing this message.)

Jason