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Subject: Re: [PATCH 5/5] x86/speculation: Add PSF mitigation kernel parameters
To:     Ramakrishna Saripalli <rsaripal@amd.com>,
        linux-kernel@vger.kernel.org, x86@kernel.org, tglx@linutronix.de,
        mingo@redhat.com, bp@alien8.de, Jonathan Corbet <corbet@lwn.net>
References: <20210407125024.242491-1-rsaripal@amd.com>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <72b45d78-f66b-bd18-4931-04feb206cd79@infradead.org>
Date:   Thu, 8 Apr 2021 20:43:07 -0700
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On 4/7/21 5:50 AM, Ramakrishna Saripalli wrote:
> From: Ramakrishna Saripalli <rk.saripalli@amd.com>
> 
> PSF mitigation introduces new kernel parameters.
> 
> The kernel parameters for PSF mitigation are modeled
> after spec_store_bypass_disable.

Maybe too much copy-pasta. See below.

> 
> Signed-off-by: Ramakrishna Saripalli<rk.saripalli@amd.com>
> ---
>  .../admin-guide/kernel-parameters.txt         | 45 +++++++++++++++++++
>  1 file changed, 45 insertions(+)
> 
> diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
> index 04545725f187..68dfde77a87d 100644
> --- a/Documentation/admin-guide/kernel-parameters.txt
> +++ b/Documentation/admin-guide/kernel-parameters.txt
> @@ -2876,6 +2876,7 @@
>  					       nospectre_v2 [X86,PPC,S390,ARM64]
>  					       spectre_v2_user=off [X86]
>  					       spec_store_bypass_disable=off [X86,PPC]
> +					       psfd=off [X86]
>  					       ssbd=force-off [ARM64]
>  					       l1tf=off [X86]
>  					       mds=off [X86]
> @@ -3243,6 +3244,8 @@
>  
>  	nohugeiomap	[KNL,X86,PPC,ARM64] Disable kernel huge I/O mappings.
>  
> +	nopsfd          [HW,X86] Disable mitigation for Predictive Store Forwarding.
> +
>  	nosmt		[KNL,S390] Disable symmetric multithreading (SMT).
>  			Equivalent to smt=1.
>  
> @@ -4002,6 +4005,48 @@
>  			that).
>  			Format: <bool>
>  
> +        psfd=		[HW,X86]
> +                        Predictive Store Forwarding Disable control
> +
> +                        Certain AMD processors feature a new technology called Predictive
> +                        Store Forwarding. This feature is designed to improve the
> +                        performance of code execution by predicting dependencies
> +                        between loads and stores.
> +
> +                        Modern processors implement techniques to optimize the
> +                        execution of a load instruction to an address that was
> +                        recently written by a store instruction.
> +
> +                        PSF expands on the above by speculating on the relationship
> +                        between loads and stores without waiting for address
> +                        calculation to complete. With PSF, CPU learns over time the
> +                        relationship between loads and stores.
> +
> +                        Incorrect PSF predictions can occur for various reasons.
> +                        Please see the AMD PSF whitepaper for more information.
> +
> +                        All AMD processors that implement PSF also provide ability
> +                        to control mitigation of PSF.
> +
> +                        Following options are provided to control PSF mitigation.
> +
> +                        The options are:
> +                        on      - Unconditionally disable Speculative Store Bypass

			                                     PSF.

> +                        off     - Unconditionally enable Speculative Store Bypass

			                                    PSF.

> +                        auto    - Kernel detects whether the CPU is vulnerable.
> +                                  If the CPU is not vulnerable, off is selected.
> +                                  If the CPU is vulnerable, default mitigation is
> +                                  KConfig dependent.
> +                        prctl   - Control Predictive Store Forwarding per thread
> +                                  via prctl. Predictive Store Forwarding is enabled
> +                                  per process by default. The state of the control
> +                                  is inherited on fork.
> +                        seccomp - Same as prctl above but all seccomp threads will
> +                                  disable PSF unless they opt out.
> +
> +                        Default mitigations:
> +                        [X86] If CONFIG_SECCOMP=y "seccomp" else "prctl"
> +
>  	psi=		[KNL] Enable or disable pressure stall information
>  			tracking.
>  			Format: <bool>
> 


-- 
~Randy