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

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Message-ID: <20251104132118.GCaQn9zoT_sqwHeX-4@fat_crate.local>
Date: Tue, 4 Nov 2025 14:21:18 +0100
From: Borislav Petkov <bp@...en8.de>
To: Christopher Snowhill <chris@...e54.net>
Cc: "Jason A. Donenfeld" <Jason@...c4.com>,
	Gregory Price <gourry@...rry.net>, 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 Mon, Nov 03, 2025 at 03:55:53PM -0800, Christopher Snowhill wrote:
> https://lore.kernel.org/lkml/9a27f2e6-4f62-45a6-a527-c09983b8dce4@cachyos.org/

tglx already summed up what the options are:

https://lore.kernel.org/all/878qgnw0vt.ffs@tglx

> Qt is built with -march=znver4, which automatically enables -mrdseed.
> This is building rdseed 64 bit, but then the software is also performing
> kernel feature checks on startup. There is no separate feature flag for
> 16/32/64 variants.

No, there aren't.

And the problem here is that, AFAICT, Qt is not providing a proper fallback
for !RDSEED. Dunno, maybe getrandom(2) or so. It is only a syscall which has
been there since forever. Rather, it would simply throw hands in the air.

Soon there will be client microcode fixes too so all should be well.

-- 
Regards/Gruss,
    Boris.

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