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

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Date:   Wed, 10 Jan 2018 00:09:47 +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:
> On 1/9/2018 4:46 PM, Thomas Gleixner wrote:
> > Urgh. That's an awful hack. why not do the obvious?
> 
> My first attempt was very similar to your change below, but testing
> showed that spectre_v2_check_boottime_disable() is called before the
> X86_FEATURE_LFENCE_RDTSC can be set.  I can look at moving where the
> X86_FEATURE_LFENCE_RDTSC is set, maybe into early_init_amd() or such
> if you think that would be best.

Wait, we can move the selection _AFTER_ identify_boot_cpu().

Thanks,

	tglx

8<----------------

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -152,7 +152,5 @@
 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 #endif
 
-void spectre_v2_check_boottime_disable(void);
-
 #endif /* __ASSEMBLY__ */
 #endif /* __NOSPEC_BRANCH_H__ */
--- a/arch/x86/kernel/cpu/bugs.c
+++ b/arch/x86/kernel/cpu/bugs.c
@@ -24,6 +24,8 @@
 #include <asm/pgtable.h>
 #include <asm/set_memory.h>
 
+static void __init spectre_v2_check_boottime_disable(void);
+
 void __init check_bugs(void)
 {
 	identify_boot_cpu();
@@ -33,6 +35,9 @@ void __init check_bugs(void)
 		print_cpu_info(&boot_cpu_data);
 	}
 
+	/* Select the proper spectre mitigation before patching alternatives */
+	spectre_v2_check_boottime_disable();
+
 #ifdef CONFIG_X86_32
 	/*
 	 * Check whether we are able to run this kernel safely on SMP.
@@ -106,7 +111,7 @@ static inline bool match_option(const ch
 	return len == arglen && !strncmp(arg, opt, len);
 }
 
-void __init spectre_v2_check_boottime_disable(void)
+static void __init spectre_v2_check_boottime_disable(void)
 {
 	char arg[20];
 	int ret;
@@ -148,14 +153,21 @@ 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
--- a/arch/x86/kernel/setup.c
+++ b/arch/x86/kernel/setup.c
@@ -1322,8 +1322,6 @@ void __init setup_arch(char **cmdline_p)
 
 	register_refined_jiffies(CLOCK_TICK_RATE);
 
-	spectre_v2_check_boottime_disable();
-
 #ifdef CONFIG_EFI
 	if (efi_enabled(EFI_BOOT))
 		efi_apply_memmap_quirks();