[PATCH v6 2/4] MIPS: CPS: Add VP(E) stealing

[Matt Redfearn]

From: Lisa Parratt <Lisa.Parratt@xxxxxxxxxx>

VP(E) stealing provides a mechanism for removing an offline Virtual
Processor from the Linux kernel such that it is available to run bare
metal code.
Once the CPU has been offlined from Linux, the CPU can be given a task
to run via mips_cps_steal_cpu_and_execute(). The CPU is removed from the
cpu_present mask and is set up to execute from address entry_fn. Stack
space is assigned via the tsk task_struct so that C initialisation code
may be used.
To return the CPU back to Linux control, mips_cps_halt_and_return_cpu
will arrange to halt the CPU and return it to the cpu_present mask. It
is then available to be brought online again via CPU hotplug.

This mechanism is used by the MIPS remote processor driver to allow
CPUs within the system to execute bare metal code, not under control of
the kernel.

Signed-off-by: Lisa Parratt <Lisa.Parratt@xxxxxxxxxx>
Signed-off-by: Matt Redfearn <matt.redfearn@xxxxxxxxxx>
---

Changes in v6: None
Changes in v5: None
Changes in v4: None
Changes in v3: None
Changes in v2: None

 arch/mips/Kconfig               |   7 ++
 arch/mips/include/asm/smp-cps.h |   8 ++
 arch/mips/kernel/smp-cps.c      | 162 ++++++++++++++++++++++++++++++++++++++--
 arch/mips/kernel/smp.c          |  12 +++
 4 files changed, 182 insertions(+), 7 deletions(-)

diff --git a/arch/mips/Kconfig b/arch/mips/Kconfig
index a008a9f03072..6cdc23da8761 100644
--- a/arch/mips/Kconfig
+++ b/arch/mips/Kconfig
@@ -2381,6 +2381,13 @@ config MIPS_CPS
 	  no external assistance. It is safe to enable this when hardware
 	  support is unavailable.
 
+config MIPS_CPU_STEAL
+	bool "VPE stealing"
+	depends on HOTPLUG_CPU && MIPS_CPS
+	help
+	  Select this is you wish to be able to run bare metal code on offline
+	  VPEs.
+
 config MIPS_CPS_PM
 	depends on MIPS_CPS
 	select MIPS_CPC
diff --git a/arch/mips/include/asm/smp-cps.h b/arch/mips/include/asm/smp-cps.h
index 2ae1f61a4a95..5cab043d7e6f 100644
--- a/arch/mips/include/asm/smp-cps.h
+++ b/arch/mips/include/asm/smp-cps.h
@@ -34,6 +34,14 @@ extern void mips_cps_boot_vpes(struct core_boot_config *cfg, unsigned vpe);
 extern void mips_cps_pm_save(void);
 extern void mips_cps_pm_restore(void);
 
+#ifdef CONFIG_MIPS_CPU_STEAL
+
+extern int mips_cps_steal_cpu_and_execute(unsigned int cpu, void *entry_fn,
+					  struct task_struct *tsk);
+extern int mips_cps_halt_and_return_cpu(unsigned int cpu);
+
+#endif /* CONFIG_MIPS_CPU_STEAL */
+
 #ifdef CONFIG_MIPS_CPS
 
 extern bool mips_cps_smp_in_use(void);
diff --git a/arch/mips/kernel/smp-cps.c b/arch/mips/kernel/smp-cps.c
index 6d45f05538c8..3a1e08a0c327 100644
--- a/arch/mips/kernel/smp-cps.c
+++ b/arch/mips/kernel/smp-cps.c
@@ -8,6 +8,7 @@
  * option) any later version.
  */
 
+#include <linux/cpu.h>
 #include <linux/delay.h>
 #include <linux/io.h>
 #include <linux/irqchip/mips-gic.h>
@@ -40,6 +41,31 @@ static int __init setup_nothreads(char *s)
 }
 early_param("nothreads", setup_nothreads);
 
+#ifdef CONFIG_MIPS_CPU_STEAL
+struct cpumask cpu_stolen_mask;
+
+static inline bool cpu_stolen(int cpu)
+{
+	return cpumask_test_cpu(cpu, &cpu_stolen_mask);
+}
+
+static inline void set_cpu_stolen(int cpu, bool state)
+{
+	if (state)
+		cpumask_set_cpu(cpu, &cpu_stolen_mask);
+	else
+		cpumask_clear_cpu(cpu, &cpu_stolen_mask);
+}
+#else
+static inline bool cpu_stolen(int cpu)
+{
+	return false;
+}
+
+static inline void set_cpu_stolen(int cpu, bool state) { }
+
+#endif /* CONFIG_MIPS_CPU_STEAL */
+
 static unsigned core_vpe_count(unsigned core)
 {
 	unsigned cfg;
@@ -110,6 +136,10 @@ static void __init cps_smp_setup(void)
 		write_gcr_bev_base(core_entry);
 	}
 
+#ifdef CONFIG_MIPS_CPU_STEAL
+	cpumask_clear(&cpu_stolen_mask);
+#endif /* CONFIG_MIPS_CPU_STEAL */
+
 #ifdef CONFIG_MIPS_MT_FPAFF
 	/* If we have an FPU, enroll ourselves in the FPU-full mask */
 	if (cpu_has_fpu)
@@ -288,7 +318,7 @@ static void remote_vpe_boot(void *dummy)
 	mips_cps_boot_vpes(core_cfg, cpu_vpe_id(&current_cpu_data));
 }
 
-static void cps_boot_secondary(int cpu, struct task_struct *idle)
+static void cps_start_secondary(int cpu, void *entry_fn, struct task_struct *tsk)
 {
 	unsigned core = cpu_data[cpu].core;
 	unsigned vpe_id = cpu_vpe_id(&cpu_data[cpu]);
@@ -298,9 +328,9 @@ static void cps_boot_secondary(int cpu, struct task_struct *idle)
 	unsigned int remote;
 	int err;
 
-	vpe_cfg->pc = (unsigned long)&smp_bootstrap;
-	vpe_cfg->sp = __KSTK_TOS(idle);
-	vpe_cfg->gp = (unsigned long)task_thread_info(idle);
+	vpe_cfg->pc = (unsigned long)entry_fn;
+	vpe_cfg->sp = __KSTK_TOS(tsk);
+	vpe_cfg->gp = (unsigned long)task_thread_info(tsk);
 
 	atomic_or(1 << cpu_vpe_id(&cpu_data[cpu]), &core_cfg->vpe_mask);
 
@@ -348,6 +378,11 @@ static void cps_boot_secondary(int cpu, struct task_struct *idle)
 	preempt_enable();
 }
 
+static void cps_boot_secondary(int cpu, struct task_struct *idle)
+{
+	cps_start_secondary(cpu, &smp_bootstrap, idle);
+}
+
 static void cps_init_secondary(void)
 {
 	/* Disable MT - we only want to run 1 TC per VPE */
@@ -399,6 +434,28 @@ static int cps_cpu_disable(void)
 	if (!cps_pm_support_state(CPS_PM_POWER_GATED))
 		return -EINVAL;
 
+#ifdef CONFIG_MIPS_CPU_STEAL
+	/*
+	 * With the MT ASE only VPEs in the same core may read / write the
+	 * control registers of other VPEs. Therefore to maintain control of
+	 * any stolen VPEs at least one sibling VPE must be kept online.
+	 */
+	if (cpu_has_mipsmt) {
+		int stolen, siblings = 0;
+
+		for_each_cpu((stolen), &cpu_stolen_mask)
+			if (cpu_data[stolen].core == cpu_data[cpu].core)
+				siblings++;
+
+		if (siblings == 1)
+			/*
+			 * When a VPE has been stolen, keep at least one of it's
+			 * siblings around in order to control it.
+			 */
+			return -EBUSY;
+	}
+#endif /* CONFIG_MIPS_CPU_STEAL */
+
 	core_cfg = &mips_cps_core_bootcfg[current_cpu_data.core];
 	atomic_sub(1 << cpu_vpe_id(&current_cpu_data), &core_cfg->vpe_mask);
 	smp_mb__after_atomic();
@@ -430,7 +487,7 @@ void play_dead(void)
 		core = cpu_data[cpu].core;
 
 		/* Look for another online VPE within the core */
-		for_each_online_cpu(cpu_death_sibling) {
+		for_each_possible_cpu(cpu_death_sibling) {
 			if (cpu_data[cpu_death_sibling].core != core)
 				continue;
 
@@ -438,8 +495,11 @@ void play_dead(void)
 			 * There is an online VPE within the core. Just halt
 			 * this TC and leave the core alone.
 			 */
-			cpu_death = CPU_DEATH_HALT;
-			break;
+			if (cpu_online(cpu_death_sibling) ||
+			    cpu_stolen(cpu_death_sibling))
+				cpu_death = CPU_DEATH_HALT;
+			if (cpu_online(cpu_death_sibling))
+				break;
 		}
 	}
 
@@ -470,6 +530,94 @@ void play_dead(void)
 	panic("Failed to offline CPU %u", cpu);
 }
 
+#ifdef CONFIG_MIPS_CPU_STEAL
+
+/* Find an online sibling CPU (another VPE in the same core) */
+static inline int mips_cps_get_online_sibling(unsigned int cpu)
+{
+	int sibling;
+
+	for_each_online_cpu(sibling)
+		if (cpu_data[sibling].core == cpu_data[cpu].core)
+			return sibling;
+
+	return -1;
+}
+
+int mips_cps_steal_cpu_and_execute(unsigned int cpu, void *entry_fn,
+				   struct task_struct *tsk)
+{
+	int err = -EINVAL;
+
+	preempt_disable();
+
+	if (!cpu_present(cpu) || cpu_online(cpu) || cpu_stolen(cpu))
+		goto out;
+
+	if (cpu_has_mipsmt && (mips_cps_get_online_sibling(cpu) < 0))
+		pr_warn("CPU%d has no online siblings to control it\n", cpu);
+	else {
+		set_cpu_present(cpu, false);
+		set_cpu_stolen(cpu, true);
+
+		cps_start_secondary(cpu, entry_fn, tsk);
+		err = 0;
+	}
+out:
+	preempt_enable();
+	return err;
+}
+
+static void mips_cps_halt_sibling(void *ptr_cpu)
+{
+	unsigned int cpu = (unsigned long)ptr_cpu;
+	unsigned int vpe_id = cpu_vpe_id(&cpu_data[cpu]);
+	unsigned long flags;
+	int vpflags;
+
+	local_irq_save(flags);
+	vpflags = dvpe();
+	settc(vpe_id);
+	write_tc_c0_tchalt(TCHALT_H);
+	evpe(vpflags);
+	local_irq_restore(flags);
+}
+
+int mips_cps_halt_and_return_cpu(unsigned int cpu)
+{
+	unsigned int core = cpu_data[cpu].core;
+	unsigned int vpe_id = cpu_vpe_id(&cpu_data[cpu]);
+
+	if (!cpu_stolen(cpu))
+		return -EINVAL;
+
+	if (cpu_has_mipsmt && (core == cpu_data[smp_processor_id()].core))
+		mips_cps_halt_sibling((void *)(unsigned long)cpu);
+	else if (cpu_has_mipsmt) {
+		int sibling = mips_cps_get_online_sibling(cpu);
+
+		if (sibling < 0) {
+			pr_warn("CPU%d has no online siblings\n", cpu);
+			return -EINVAL;
+		}
+
+		if (smp_call_function_single(sibling, mips_cps_halt_sibling,
+						(void *)(unsigned long)cpu, 1))
+			panic("Failed to call sibling CPU\n");
+
+	} else if (cpu_has_vp) {
+		mips_cm_lock_other(core, vpe_id);
+		write_cpc_co_vp_stop(1 << vpe_id);
+		mips_cm_unlock_other();
+	}
+
+	set_cpu_stolen(cpu, false);
+	set_cpu_present(cpu, true);
+	return 0;
+}
+
+#endif /* CONFIG_MIPS_CPU_STEAL */
+
 static void wait_for_sibling_halt(void *ptr_cpu)
 {
 	unsigned cpu = (unsigned long)ptr_cpu;
diff --git a/arch/mips/kernel/smp.c b/arch/mips/kernel/smp.c
index 6e71130549ea..ae1ffc3e33eb 100644
--- a/arch/mips/kernel/smp.c
+++ b/arch/mips/kernel/smp.c
@@ -235,6 +235,18 @@ static void smp_ipi_init_one(unsigned int virq,
 				    struct irqaction *action)
 {
 	int ret;
+#ifdef CONFIG_MIPS_CPU_STEAL
+	struct irq_data *data;
+	/*
+	 * A bit of a hack to ensure that the ipi_offset is 0.
+	 * This is to deal with removing / reallocating IPIs
+	 * to subsets of the possible CPUs, where the IPI IRQ domain
+	 * will set ipi_offset to the first cpu in the cpumask when the
+	 * IPI is reallocated.
+	 */
+	data = irq_get_irq_data(virq);
+	data->common->ipi_offset = 0;
+#endif /* CONFIG_MIPS_CPU_STEAL */
 
 	irq_set_handler(virq, handle_percpu_irq);
 	ret = setup_irq(virq, action);
-- 
2.7.4