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Date:   Thu, 5 May 2022 10:28:24 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     "Luck, Tony" <tony.luck@intel.com>, hdegoede@redhat.com,
        markgross@kernel.org, mingo@redhat.com, bp@alien8.de,
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        corbet@lwn.net, gregkh@linuxfoundation.org,
        andriy.shevchenko@linux.intel.com, jithu.joseph@intel.com,
        ashok.raj@intel.com, rostedt@goodmis.org, dan.j.williams@intel.com,
        linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org,
        platform-driver-x86@vger.kernel.org, patches@lists.linux.dev,
        ravi.v.shankar@intel.com
Subject: Re: [PATCH v5 07/10] platform/x86/intel/ifs: Add scan test support
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On Thu, May 05, 2022 at 01:15:07AM +0200, Thomas Gleixner wrote:
> We don't have stomp_cpumask() today, but that's trivial enough to
> implement.

I don't think we want to gift people a random cpumask stop_machine(),
but here's one that stops a core. It runs the @fn on every cpu since I
thought to have understood that was the requirement for this muck.

*completely* untestededed.

diff --git a/kernel/stop_machine.c b/kernel/stop_machine.c
index 6da7b91af353..2e7324e44e38 100644
--- a/kernel/stop_machine.c
+++ b/kernel/stop_machine.c
@@ -631,6 +631,34 @@ int stop_machine(cpu_stop_fn_t fn, void *data, const struct cpumask *cpus)
 }
 EXPORT_SYMBOL_GPL(stop_machine);
 
+/*
+ * stop_core_cpuslocked - stop_machine a core
+ * @cpu: any cpu in the targeted core
+ * @fn: the function to run
+ * @data: the data ptr for @fn()
+ *
+ * RETURNS:
+ * 0 if all executions of @fn returned 0, any non zero return value if any
+ * returned non zero.
+ */
+int stop_core_cpuslocked(unsigned int cpu, cpu_stop_fn_t fn, void *data)
+{
+	const struct cpumask *smt_mask = cpu_smt_mask(cpu);
+
+	struct multi_stop_data msdata = {
+		.fn = fn,
+		.data = data,
+		.num_threads = cpumask_weight(smt_mask);
+		.active_cpus = smt_mask,
+	};
+
+	lockdep_assert_cpus_held();
+
+	/* Set the initial state and stop all online cpus. */
+	set_state(&msdata, MULTI_STOP_PREPARE);
+	return stop_cpus(smt_mask, multi_cpu_stop, &msdata);
+}
+
 /**
  * stop_machine_from_inactive_cpu - stop_machine() from inactive CPU
  * @fn: the function to run