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Date:   Fri, 21 May 2021 09:53:15 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Hugh Dickins <hughd@google.com>
Cc:     linux-kernel@vger.kernel.org, joel@joelfernandes.org
Subject: Re: config SCHED_CORE
Message-ID: <YKdm69K8k/ztd6BM@hirez.programming.kicks-ass.net>
References: <alpine.LSU.2.11.2105201954180.6100@eggly.anvils>
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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)
+	  -- 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.