linux-kernel - Re: [v6 1/1] x86/bugs: Implement mitigation for Predictive Store Forwarding

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Date:   Tue, 18 May 2021 13:35:27 -0700
From:   Pawan Gupta <writetopawan@...il.com>
To:     "Saripalli, RK" <rsaripal@....com>
Cc:     Randy Dunlap <rdunlap@...radead.org>, linux-kernel@...r.kernel.org,
        x86@...nel.org, tglx@...utronix.de, mingo@...hat.com, bp@...en8.de,
        hpa@...or.com, Jonathan Corbet <corbet@....net>, bsd@...hat.com
Subject: Re: [v6 1/1] x86/bugs: Implement mitigation for Predictive Store
 Forwarding

On 18.05.2021 07:27, Saripalli, RK wrote:
>
>
>On 5/17/2021 9:55 PM, Randy Dunlap wrote:
>> Hi again,
>>
>> On 5/17/21 3:00 PM, Ramakrishna Saripalli wrote:
>>> From: Ramakrishna Saripalli <rk.saripalli@....com>
>>>
>>> Certain AMD processors feature a new technology called Predictive Store
>>> Forwarding (PSF).
>>>
>>> PSF is a micro-architectural optimization designed to improve the
>>> performance of code execution by predicting dependencies between
>>> loads and stores.
>>>
>>> Incorrect PSF predictions can occur due to two reasons.
>>>
>> ...
>>
>>>
>>> Kernel parameter predictive_store_fwd_disable has the following values
>>>
>>> - on. Disable PSF on all CPUs.
>>>
>>> - off. Enable PSF on all CPUs.
>>>        This is also the default setting.
>>>
>>> Signed-off-by: Ramakrishna Saripalli<rk.saripalli@....com>
>>> ---
>>>  .../admin-guide/kernel-parameters.txt         |  5 +
>>>  arch/x86/include/asm/cpufeatures.h            |  1 +
>>>  arch/x86/include/asm/msr-index.h              |  2 +
>>>  arch/x86/include/asm/nospec-branch.h          |  6 ++
>>>  arch/x86/kernel/cpu/bugs.c                    | 94 +++++++++++++++++++
>>>  5 files changed, 108 insertions(+)
>>>
>>> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
>>> index 04545725f187..a5f694dccb24 100644
>>> --- a/Documentation/admin-guide/kernel-parameters.txt
>>> +++ b/Documentation/admin-guide/kernel-parameters.txt
>>> @@ -3940,6 +3940,11 @@
>>>  			Format: {"off"}
>>>  			Disable Hardware Transactional Memory
>>>
>>> +	predictive_store_fwd_disable=	[X86] This option controls PSF.
>>> +			off - Turns on PSF.
>>> +			on  - Turns off PSF.
>>> +			default : off.
>>
>>
>> and as I did earlier, I still object to "off" meaning PSF is on
>> and "on" meaning that PSF is off.
>>
>> It's not at all user friendly.
>>
>> If it's done this way because that's how the h/w bit is defined/used,
>> that's not a good excuse IMHO.
>>
>> Hm, it sorta seems to be a common "theme" when dealing with mitigations.
>> And too late to fix that.
>
>Based on previous feedback from Thomas Gleixner, I reworded this as a mitigation instead of as a "feature".
>In that vein, all the mitigation code moved into bugs.c like other mitigations, similar to
>spec_control_bypass_disable with an ON/OFF but no prctl/seccomp/auto.

Maybe change the help text to something like:

	on  - Turns on PSF mitigation.
	off - Turns off PSF mitigation.

Thanks,
Pawan