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

From: Thomas Gleixner
Date: Tue Jan 09 2018 - 17:47:14 EST


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@xxxxxxx>
> ---
> 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