linux-kernel - Re: [PATCH v3 12/35] x86/bugs: Restructure retbleed mitigation

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Message-ID: <20250224154529.GJZ7yUGfQiyQJc_aMl@fat_crate.local>
Date: Mon, 24 Feb 2025 16:45:29 +0100
From: Borislav Petkov <bp@...en8.de>
To: David Kaplan <david.kaplan@....com>
Cc: Thomas Gleixner <tglx@...utronix.de>,
	Peter Zijlstra <peterz@...radead.org>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
	Ingo Molnar <mingo@...hat.com>,
	Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
	"H . Peter Anvin" <hpa@...or.com>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 12/35] x86/bugs: Restructure retbleed mitigation

On Wed, Jan 08, 2025 at 02:24:52PM -0600, David Kaplan wrote:
> +static void __init retbleed_update_mitigation(void)
> +{
> +	if (!boot_cpu_has_bug(X86_BUG_RETBLEED) || cpu_mitigations_off())
> +		return;
>  
> -		break;
> +	if (retbleed_mitigation == RETBLEED_MITIGATION_NONE)
> +		goto out;
> +	/*
> +	 * Let IBRS trump all on Intel without affecting the effects of the
> +	 * retbleed= cmdline option except for call depth based stuffing
> +	 */
> +	if (boot_cpu_data.x86_vendor == X86_VENDOR_INTEL) {
> +		switch (spectre_v2_enabled) {
> +		case SPECTRE_V2_IBRS:
> +			retbleed_mitigation = RETBLEED_MITIGATION_IBRS;
> +			break;
> +		case SPECTRE_V2_EIBRS:
> +		case SPECTRE_V2_EIBRS_RETPOLINE:
> +		case SPECTRE_V2_EIBRS_LFENCE:
> +			retbleed_mitigation = RETBLEED_MITIGATION_EIBRS;
> +			break;
> +		default:
> +			if (retbleed_mitigation != RETBLEED_MITIGATION_STUFF)
> +				pr_err(RETBLEED_INTEL_MSG);
> +		}
>  	}
>  
> +	if (retbleed_mitigation == RETBLEED_MITIGATION_STUFF) {
> +		if (spectre_v2_enabled != SPECTRE_V2_RETPOLINE) {
> +			pr_err("WARNING: retbleed=stuff depends on spectre_v2=retpoline\n");
> +			retbleed_mitigation = RETBLEED_MITIGATION_AUTO;
> +			/* Try again */
> +			retbleed_select_mitigation();

Err, why?

spectre_v2 and spectre_v2_enabled cannot change anymore - the select function
has set them. Why try again here?

This kinda defeats the whole purpose of having the select -> update -> apply
rounds...

-- 
Regards/Gruss,
    Boris.

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