[PATCH v3 3/4] cpufreq: ACPI: Use IPI to update boost MSR in cpufreq_boost_down_prep()

[Jim Mattson <jmattson@google.com>]

During driver exit or CPU hotplug, acpi_cpufreq_cpu_exit() calls
cpufreq_boost_down_prep() to re-enable boost on the target CPU.  However,
cpufreq_boost_down_prep() ignores the target CPU parameter and calls
boost_set_msr(1) locally. Since this runs in an unbound process context, it
updates the local CPU's MSR while leaving the target CPU's boost state
unchanged.

On Intel platforms, the open-coded read-modify-write of
MSR_IA32_MISC_ENABLE in boost_set_msr() is vulnerable to preemption and
thread migration if executed in process context.

Fix both issues by routing the MSR update through
smp_call_function_single() to execute boost_set_msr() synchronously on the
target CPU. This ensures the correct CPU is updated and that the RMW
sequence on Intel executes safely in IPI context with interrupts disabled.

Fixes: a3605c46e0c0 ("cpufreq: acpi-cpufreq: drop rdmsr_on_cpus() usage")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Closes: https://sashiko.dev/#/patchset/20260612215729.1532175-1-jmattson%40google.com
Assisted-by: Gemini:gemini-3.5-pro
Signed-off-by: Jim Mattson <jmattson@google.com>
---
 drivers/cpufreq/acpi-cpufreq.c | 7 ++++---
 1 file changed, 4 insertions(+), 3 deletions(-)

diff --git a/drivers/cpufreq/acpi-cpufreq.c b/drivers/cpufreq/acpi-cpufreq.c
index 21639d9ac753..48d678812fd6 100644
--- a/drivers/cpufreq/acpi-cpufreq.c
+++ b/drivers/cpufreq/acpi-cpufreq.c
@@ -528,10 +528,11 @@ static void free_acpi_perf_data(void)
 static int cpufreq_boost_down_prep(unsigned int cpu)
 {
 	/*
-	 * Clear the boost-disable bit on the CPU_DOWN path so that
-	 * this cpu cannot block the remaining ones from boosting.
+	 * Clear the boost-disable bit on the target CPU so that
+	 * it cannot block the remaining ones from boosting.
 	 */
-	return boost_set_msr(1);
+	return smp_call_function_single(cpu, boost_set_msr_each,
+					(void *)1L, 1);
 }
 
 /*
-- 
2.55.0.rc0.799.gd6f94ed593-goog