linux-kernel - Re: [tip: x86/urgent] x86/speculation: Avoid force-disabling IBPB based on STIBP and enhanced IBRS.

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Date:   Thu, 11 Jun 2020 16:09:51 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Anthony Steinhauser <asteinhauser@...gle.com>
Cc:     linux-tip-commits@...r.kernel.org,
        Anthony Steinhauser <asteinhauser@...gle.com>,
        Thomas Gleixner <tglx@...utronix.de>, stable@...r.kernel.org,
        x86 <x86@...nel.org>, LKML <linux-kernel@...r.kernel.org>
Subject: Re: [tip: x86/urgent] x86/speculation: Avoid force-disabling IBPB
 based on STIBP and enhanced IBRS.

On Tue, Jun 09, 2020 at 08:53:49AM -0000, tip-bot2 for Anthony Steinhauser wrote:
> @@ -672,23 +665,36 @@ spectre_v2_user_select_mitigation(enum spectre_v2_mitigation_cmd v2_cmd)
>  		pr_info("mitigation: Enabling %s Indirect Branch Prediction Barrier\n",
>  			static_key_enabled(&switch_mm_always_ibpb) ?
>  			"always-on" : "conditional");
> +
> +		spectre_v2_user_ibpb = mode;
>  	}
>  
> -	/* If enhanced IBRS is enabled no STIBP required */
> -	if (spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
> +	/*
> +	 * If enhanced IBRS is enabled or SMT impossible, STIBP is not
> +	 * required.
> +	 */
> +	if (!smt_possible || spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
>  		return;
>  
>  	/*
> -	 * If SMT is not possible or STIBP is not available clear the STIBP
> -	 * mode.
> +	 * At this point, an STIBP mode other than "off" has been set.
> +	 * If STIBP support is not being forced, check if STIBP always-on
> +	 * is preferred.
> +	 */
> +	if (mode != SPECTRE_V2_USER_STRICT &&
> +	    boot_cpu_has(X86_FEATURE_AMD_STIBP_ALWAYS_ON))
> +		mode = SPECTRE_V2_USER_STRICT_PREFERRED;
> +
> +	/*
> +	 * If STIBP is not available, clear the STIBP mode.
>  	 */
> -	if (!smt_possible || !boot_cpu_has(X86_FEATURE_STIBP))
> +	if (!boot_cpu_has(X86_FEATURE_STIBP))
>  		mode = SPECTRE_V2_USER_NONE;

Can we merge this test into the one above? Diff ontop:

---
diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
index 8d57562b1d2c..05b3163e1b8c 100644
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -673,7 +673,9 @@ spectre_v2_user_select_mitigation(enum spectre_v2_mitigation_cmd v2_cmd)
 	 * If enhanced IBRS is enabled or SMT impossible, STIBP is not
 	 * required.
 	 */
-	if (!smt_possible || spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
+	if (!boot_cpu_has(X86_FEATURE_STIBP) ||
+	    !smt_possible ||
+	    spectre_v2_enabled == SPECTRE_V2_IBRS_ENHANCED)
 		return;
 
 	/*
@@ -685,12 +687,6 @@ spectre_v2_user_select_mitigation(enum spectre_v2_mitigation_cmd v2_cmd)
 	    boot_cpu_has(X86_FEATURE_AMD_STIBP_ALWAYS_ON))
 		mode = SPECTRE_V2_USER_STRICT_PREFERRED;
 
-	/*
-	 * If STIBP is not available, clear the STIBP mode.
-	 */
-	if (!boot_cpu_has(X86_FEATURE_STIBP))
-		mode = SPECTRE_V2_USER_NONE;
-
 	spectre_v2_user_stibp = mode;
 
 set_mode:

-- 
Regards/Gruss,
    Boris.

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