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Subject: Re: config SCHED_CORE
To:     Peter Zijlstra <peterz@infradead.org>,
        Hugh Dickins <hughd@google.com>
Cc:     linux-kernel@vger.kernel.org, joel@joelfernandes.org
References: <alpine.LSU.2.11.2105201954180.6100@eggly.anvils>
 <YKdm69K8k/ztd6BM@hirez.programming.kicks-ass.net>
From:   Randy Dunlap <rdunlap@infradead.org>
Message-ID: <47848ac4-6164-57fe-de16-7ca6048aa371@infradead.org>
Date:   Fri, 21 May 2021 09:09:54 -0700
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On 5/21/21 12:53 AM, Peter Zijlstra wrote:
> On Thu, May 20, 2021 at 08:06:07PM -0700, Hugh Dickins wrote:
>> Hi Peter,
>>
>> make oldconfig gave me no help at all on how to decide whether to choose
>> SCHED_CORE Y or n, beyond it recommending Y.  Maybe you'll delete that
>> option later, or maybe removing the prompt string would silence it.
> 
> Ah, you're quite right. I never seem to have gotten around to actually
> writing anything useful there :/ Similarly the documentation for all
> this seems to have gone missing too.
> 
> Joel, could I ask you to refresh the document to match the current state
> of things and repost? I still whole hartedly despise this RST crud, it
> makes it so hard to read / modify the files.
> 
> ( I think the latest version is here:
>   https://lkml.kernel.org/r/20210324214020.34142-8-joel@joelfernandes.org
> )
> 
> Anyway, how is something like the below, Joel can add a reference to the
> document once it's there.
> 
> ---
>  kernel/Kconfig.preempt | 14 +++++++++++++-
>  1 file changed, 13 insertions(+), 1 deletion(-)
> 
> diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
> index ea1e3331c0ba..3c4566cd20ef 100644
> --- a/kernel/Kconfig.preempt
> +++ b/kernel/Kconfig.preempt
> @@ -104,4 +104,16 @@ config SCHED_CORE
>  	bool "Core Scheduling for SMT"
>  	default y
>  	depends on SCHED_SMT
> -
> +	help
> +	  This option enables Core scheduling, a means of coordinated task
> +	  selection across SMT siblings with the express purpose of creating a
> +	  Core wide privilidge boundary. When enabled -- see prctl(PR_SCHED_CORE)

	            privilege
while you are at it.

> +	  -- task selection will ensure all SMT siblings will execute a task
> +	  from the same 'core group', forcing idle when no matching task is found.
> +
> +	  This provides means of mitigation against a number of SMT side-channels;
> +	  but is, on its own, insufficient to mitigate all known side-channels.
> +	  Notable: the MDS class of attacks require more.
> +
> +	  Default enabled for anything that has SCHED_SMT, when unused there should
> +	  be no impact on performance.
> 


-- 
~Randy