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

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Date:   Mon, 17 May 2021 19:55:53 -0700
From:   Randy Dunlap <rdunlap@...radead.org>
To:     Ramakrishna Saripalli <rsaripal@....com>,
        linux-kernel@...r.kernel.org, x86@...nel.org, tglx@...utronix.de,
        mingo@...hat.com, bp@...en8.de, hpa@...or.com,
        Jonathan Corbet <corbet@....net>
Cc:     bsd@...hat.com
Subject: Re: [v6 1/1] x86/bugs: Implement mitigation for Predictive Store
 Forwarding

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.

I look forward to h/w that doesn't need mitigations.  ;)

-- 
~Randy