[RFC PATCH 2/7] arm64: kernel: Add a WFI hook.

[Mohamed Mediouni]

From: Stan Skowronek <s...@corellium.com>

WFI drops register state on Apple Silicon for SMP systems.

This hook will be used for a hardware workaround in the
Apple CPU start driver.

Signed-off-by: Stan Skowronek <s...@corellium.com>
Signed-off-by: Mohamed Mediouni <mohamed.medio...@caramail.com>
---
 arch/arm64/include/asm/cpu_ops.h |  2 ++
 arch/arm64/kernel/cpu_ops.c      |  6 ++++++
 arch/arm64/kernel/process.c      | 11 +++++++++--
 3 files changed, 17 insertions(+), 2 deletions(-)

diff --git a/arch/arm64/include/asm/cpu_ops.h b/arch/arm64/include/asm/cpu_ops.h
index e95c4df83911..4be0fc5bcaf9 100644
--- a/arch/arm64/include/asm/cpu_ops.h
+++ b/arch/arm64/include/asm/cpu_ops.h
@@ -23,6 +23,7 @@
  * @cpu_boot:  Boots a cpu into the kernel.
  * @cpu_postboot: Optionally, perform any post-boot cleanup or necessary
  *             synchronisation. Called from the cpu being booted.
+ * @cpu_wfi:    Optionally, replace calls to WFI in default idle with this.
  * @cpu_can_disable: Determines whether a CPU can be disabled based on
  *             mechanism-specific information.
  * @cpu_disable: Prepares a cpu to die. May fail for some mechanism-specific
@@ -43,6 +44,7 @@ struct cpu_operations {
        int             (*cpu_prepare)(unsigned int);
        int             (*cpu_boot)(unsigned int);
        void            (*cpu_postboot)(void);
+       void            (*cpu_wfi)(void);
 #ifdef CONFIG_HOTPLUG_CPU
        bool            (*cpu_can_disable)(unsigned int cpu);
        int             (*cpu_disable)(unsigned int cpu);
diff --git a/arch/arm64/kernel/cpu_ops.c b/arch/arm64/kernel/cpu_ops.c
index e133011f64b5..6979fc4490b2 100644
--- a/arch/arm64/kernel/cpu_ops.c
+++ b/arch/arm64/kernel/cpu_ops.c
@@ -19,12 +19,18 @@ extern const struct cpu_operations smp_spin_table_ops;
 extern const struct cpu_operations acpi_parking_protocol_ops;
 #endif
 extern const struct cpu_operations cpu_psci_ops;
+#ifdef CONFIG_ARCH_APPLE
+extern const struct cpu_operations cpu_apple_start_ops;
+#endif

 static const struct cpu_operations *cpu_ops[NR_CPUS] __ro_after_init;

 static const struct cpu_operations *const dt_supported_cpu_ops[] __initconst = 
{
        &smp_spin_table_ops,
        &cpu_psci_ops,
+#ifdef CONFIG_ARCH_APPLE
+       &cpu_apple_start_ops,
+#endif
        NULL,
 };

diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
index 34ec400288d0..611c639e20be 100644
--- a/arch/arm64/kernel/process.c
+++ b/arch/arm64/kernel/process.c
@@ -57,6 +57,7 @@
 #include <asm/processor.h>
 #include <asm/pointer_auth.h>
 #include <asm/stacktrace.h>
+#include <asm/cpu_ops.h>

 #if defined(CONFIG_STACKPROTECTOR) && !defined(CONFIG_STACKPROTECTOR_PER_TASK)
 #include <linux/stackprotector.h>
@@ -74,8 +75,14 @@ void (*arm_pm_restart)(enum reboot_mode reboot_mode, const 
char *cmd);

 static void noinstr __cpu_do_idle(void)
 {
-       dsb(sy);
-       wfi();
+       const struct cpu_operations *ops = get_cpu_ops(task_cpu(current));
+
+       if (ops->cpu_wfi) {
+               ops->cpu_wfi();
+       } else {
+               dsb(sy);
+               wfi();
+       }
 }

 static void noinstr __cpu_do_idle_irqprio(void)
--
2.29.2