linux-kernel - Re: [PATCH v2] x86/amd: Disable RDSEED on AMD Zen5 because of an error.

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Message-ID: <20251018100314.GAaPNl4ngomUnreTbZ@fat_crate.local>
Date: Sat, 18 Oct 2025 12:03:14 +0200
From: Borislav Petkov <bp@...en8.de>
To: Gregory Price <gourry@...rry.net>
Cc: x86@...nel.org, linux-kernel@...r.kernel.org, tglx@...utronix.de,
	mingo@...hat.com, dave.hansen@...ux.intel.com, hpa@...or.com,
	peterz@...radead.org, mario.limonciello@....com, riel@...riel.com,
	yazen.ghannam@....com, me@...aill.net, kai.huang@...el.com,
	sandipan.das@....com, darwi@...utronix.de, stable@...r.kernel.org
Subject: Re: [PATCH v2] x86/amd: Disable RDSEED on AMD Zen5 because of an
 error.

On Fri, Oct 17, 2025 at 10:40:10PM -0400, Gregory Price wrote:
> Under unknown conditions, Zen5 chips running rdseed can produce
> (val=0,CF=1) over 10% of the time (when rdseed is successful).
> CF=1 indicates success, while val=0 is typically only produced
> when rdseed fails (CF=0).
> 
> This suggests there is a bug which causes rdseed to silently fail.
> 
> This was reproduced reliably by launching 2-threads per available
> core, 1-thread per for hamming on RDSEED, and 1-thread per core
> collectively eating and hammering on ~90% of memory.

Which version of RDSEED was used? 32-bit perhaps? Can you repro this with
the 64-bit version of RDSEED?

> This was observed on more than 1 Zen5 model, so it should be disabled
> for all of Zen5 until/unless a comprehensive blacklist can be built.

As I said the last time, we're working on it. Be patient pls.

Thx.

-- 
Regards/Gruss,
    Boris.

https://people.kernel.org/tglx/notes-about-netiquette