linux-kernel - Re: [PATCH v2 2/7] x86/retbleed: Simplify the =stuff checks

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Message-ID: <20250522114258.GGaC8NwmKyhKfJmyga@fat_crate.local>
Date: Thu, 22 May 2025 13:42:58 +0200
From: Borislav Petkov <bp@...en8.de>
To: Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>
Cc: x86@...nel.org, David Kaplan <david.kaplan@....com>,
	linux-kernel@...r.kernel.org, "H. Peter Anvin" <hpa@...or.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH v2 2/7] x86/retbleed: Simplify the =stuff checks

On Wed, May 21, 2025 at 07:44:37PM -0700, Pawan Gupta wrote:
> +	if (retbleed_mitigation == RETBLEED_MITIGATION_STUFF &&
> +	    spectre_v2_enabled != SPECTRE_V2_RETPOLINE) {
> +		pr_err("WARNING: retbleed=stuff depends on spectre_v2=retpoline\n");
> +		retbleed_mitigation = RETBLEED_MITIGATION_AUTO;

What would be the next-best thing fallback for this, short of disabling the
mitigation?

UNRET, IBPB?

I would prefer if we look at STUFFing only when SPECTRE_V2_RETPOLINE - i.e.,
is it even possible.

If not, we fallback to another mitigation which is probably more expensive but
it is better than NONE...

-- 
Regards/Gruss,
    Boris.

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