linux-kernel - Re: [PATCH v1 1/2] x86/retpoline: Add a function to clear the RETPOLINE

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Message-ID: <alpine.DEB.2.20.1801092345560.2200@nanos>
Date:   Tue, 9 Jan 2018 23:46:57 +0100 (CET)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Tom Lendacky <thomas.lendacky@....com>
cc:     x86@...nel.org, linux-kernel@...r.kernel.org,
        Rik van Riel <riel@...hat.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jiri Kosina <jikos@...nel.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Borislav Petkov <bp@...en8.de>,
        Andy Lutomirski <luto@...nel.org>,
        Kees Cook <keescook@...gle.com>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Greg Kroah-Hartman <gregkh@...ux-foundation.org>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Paul Turner <pjt@...gle.com>
Subject: Re: [PATCH v1 1/2] x86/retpoline: Add a function to clear the
 RETPOLINE_AMD feature

On Tue, 9 Jan 2018, Tom Lendacky wrote:
> For AMD hardware, the RETPOLINE_AMD feature is dependent on LFENCE being
> a serializing instruction.  Create a function to allow RETPOLINE_AMD to
> be cleared if it cannot be determined that LFENCE is serializing. In
> addition, update the spectre_v2_enabled variable so that sysfs output is
> correct.
> 
> Signed-off-by: Tom Lendacky <thomas.lendacky@....com>
> ---
>  arch/x86/include/asm/nospec-branch.h |    1 +
>  arch/x86/kernel/cpu/bugs.c           |   10 ++++++++++
>  2 files changed, 11 insertions(+)
> 
> diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
> index 8ddf851..5785684 100644
> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -153,6 +153,7 @@
>  #endif
>  
>  void spectre_v2_check_boottime_disable(void);
> +void retpoline_amd_disable(void);
>  
>  #endif /* __ASSEMBLY__ */
>  #endif /* __NOSPEC_BRANCH_H__ */
> diff --git a/arch/x86/kernel/cpu/bugs.c b/arch/x86/kernel/cpu/bugs.c
> index b957f77..a4c594c 100644
> --- a/arch/x86/kernel/cpu/bugs.c
> +++ b/arch/x86/kernel/cpu/bugs.c
> @@ -170,6 +170,16 @@ void __init spectre_v2_check_boottime_disable(void)
>  	return;
>  }
>  
> +void retpoline_amd_disable(void)
> +{
> +	if (!boot_cpu_has(X86_FEATURE_RETPOLINE_AMD))
> +		return;
> +
> +	setup_clear_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
> +	spectre_v2_enabled = retp_compiler() ?
> +		SPECTRE_V2_RETPOLINE_GENERIC : SPECTRE_V2_RETPOLINE_MINIMAL;
> +}

Urgh. That's an awful hack. why not do the obvious?

Thanks,

	tglx

--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -148,14 +148,21 @@ void __init spectre_v2_check_boottime_di
 retpoline:
 	if (boot_cpu_data.x86_vendor == X86_VENDOR_AMD) {
 	retpoline_amd:
+		if (boot_cpu_data.x86_vendor != X86_VENDOR_AMD ||
+		    !boot_cpu_has(X86_FEATURE_LFENCE_RDTSC)) {
+			pr_info("AMD retpoline not supported, fall back to generic\n");
+			goto retpoline_generic;
+		}
+
 		spectre_v2_enabled = retp_compiler() ?
 			SPECTRE_V2_RETPOLINE_AMD : SPECTRE_V2_RETPOLINE_MINIMAL_AMD;
 		setup_force_cpu_cap(X86_FEATURE_RETPOLINE_AMD);
-	} else {
-	retpoline_generic:
-		spectre_v2_enabled = retp_compiler() ?
-			SPECTRE_V2_RETPOLINE_GENERIC : SPECTRE_V2_RETPOLINE_MINIMAL;
+		setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
+		return;
 	}
+retpoline_generic:
+	spectre_v2_enabled = retp_compiler() ?
+		SPECTRE_V2_RETPOLINE_GENERIC : SPECTRE_V2_RETPOLINE_MINIMAL;
 	setup_force_cpu_cap(X86_FEATURE_RETPOLINE);
 	return;
 #else