Re: [PATCH v1 1/2] x86/retpoline: Add a function to clear the RETPOLINE_AMD feature

[Thomas Gleixner]

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.lenda...@amd.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