Subject: Re: [PATCH v1 1/2] x86/retpoline: Add a function to clear the
 RETPOLINE_AMD feature
To: Thomas Gleixner <tglx@linutronix.de>
Cc: x86@kernel.org, linux-kernel@vger.kernel.org,
        Rik van Riel <riel@redhat.com>,
        Andi Kleen <ak@linux.intel.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Jiri Kosina <jikos@kernel.org>,
        Dan Williams <dan.j.williams@intel.com>,
        Dave Hansen <dave.hansen@intel.com>,
        Borislav Petkov <bp@alien8.de>,
        Andy Lutomirski <luto@kernel.org>,
        Kees Cook <keescook@google.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Greg Kroah-Hartman <gregkh@linux-foundation.org>,
        David Woodhouse <dwmw@amazon.co.uk>,
        Paul Turner <pjt@google.com>
References: <20180109223723.29322.59126.stgit@tlendack-t1.amdoffice.net>
 <20180109223736.29322.5436.stgit@tlendack-t1.amdoffice.net>
 <alpine.DEB.2.20.1801092345560.2200@nanos>
From: Tom Lendacky <thomas.lendacky@amd.com>
Message-ID: <12147db9-e975-268f-55ee-247868e0e0d8@amd.com>
Date: Tue, 9 Jan 2018 17:01:10 -0600
User-Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:52.0) Gecko/20100101
 Thunderbird/52.5.2
MIME-Version: 1.0
In-Reply-To: <alpine.DEB.2.20.1801092345560.2200@nanos>
Content-Type: text/plain; charset=utf-8
Content-Language: en-US
Content-Transfer-Encoding: 7bit
SpamDiagnosticOutput: 1:99
SpamDiagnosticMetadata: NSPM
X-MS-Exchange-CrossTenant-OriginalArrivalTime: 09 Jan 2018 23:01:13.1783 (UTC)
X-MS-Exchange-CrossTenant-Network-Message-Id: 432fcdf0-a80a-40e8-b9d6-08d557b4e1e1
X-MS-Exchange-CrossTenant-FromEntityHeader: Hosted
X-MS-Exchange-CrossTenant-Id: 3dd8961f-e488-4e60-8e11-a82d994e183d
X-MS-Exchange-Transport-CrossTenantHeadersStamped: DM5PR12MB1147
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org
Return-Path: <linux-kernel-owner@vger.kernel.org>

On 1/9/2018 4:46 PM, Thomas Gleixner wrote:
> 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@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?

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.

Thanks,
Tom

> 
> 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
> 
>