[PATCHv2 2/4] arm64: add guest pvstate support

[Sergey Senozhatsky]

PV-vcpu-state is a per-CPU struct, which, for the time being,
holds boolean `preempted' vCPU state. During the startup,
given that host supports PV-state, each guest vCPU sends
a pointer to its per-CPU variable to the host as a payload
with the SMCCC HV call, so that host can update vCPU state
when it puts or loads vCPU.

This has impact on the guest's scheduler:

[..]
  wake_up_process()
   try_to_wake_up()
    select_task_rq_fair()
     available_idle_cpu()
      vcpu_is_preempted()

Some sched benchmarks data is available on the github page [0].

[0] https://github.com/sergey-senozhatsky/arm64-vcpu_is_preempted

Signed-off-by: Sergey Senozhatsky <senozhatsky at chromium.org>
---
 arch/arm64/include/asm/paravirt.h | 19 +++++++
 arch/arm64/kernel/paravirt.c      | 94 +++++++++++++++++++++++++++++++
 arch/arm64/kernel/smp.c           |  4 ++
 3 files changed, 117 insertions(+)

diff --git a/arch/arm64/include/asm/paravirt.h b/arch/arm64/include/asm/paravirt.h
index 9aa193e0e8f2..a3f7665dff38 100644
--- a/arch/arm64/include/asm/paravirt.h
+++ b/arch/arm64/include/asm/paravirt.h
@@ -2,6 +2,11 @@
 #ifndef _ASM_ARM64_PARAVIRT_H
 #define _ASM_ARM64_PARAVIRT_H
 
+struct vcpu_state {
+	bool	preempted;
+	u8	reserved[63];
+};
+
 #ifdef CONFIG_PARAVIRT
 #include <linux/static_call_types.h>
 
@@ -20,8 +25,22 @@ static inline u64 paravirt_steal_clock(int cpu)
 
 int __init pv_time_init(void);
 
+bool dummy_vcpu_is_preempted(unsigned int cpu);
+
+extern struct static_key pv_vcpu_is_preempted_enabled;
+DECLARE_STATIC_CALL(pv_vcpu_is_preempted, dummy_vcpu_is_preempted);
+
+static inline bool paravirt_vcpu_is_preempted(unsigned int cpu)
+{
+	return static_call(pv_vcpu_is_preempted)(cpu);
+}
+
+int __init pv_vcpu_state_init(void);
+
 #else
 
+#define pv_vcpu_state_init() do {} while (0)
+
 #define pv_time_init() do {} while (0)
 
 #endif // CONFIG_PARAVIRT
diff --git a/arch/arm64/kernel/paravirt.c b/arch/arm64/kernel/paravirt.c
index 75fed4460407..d8fc46795d94 100644
--- a/arch/arm64/kernel/paravirt.c
+++ b/arch/arm64/kernel/paravirt.c
@@ -40,6 +40,11 @@ struct pv_time_stolen_time_region {
 
 static DEFINE_PER_CPU(struct pv_time_stolen_time_region, stolen_time_region);
 
+static DEFINE_PER_CPU(struct vcpu_state, vcpus_states);
+struct static_key pv_vcpu_is_preempted_enabled;
+
+DEFINE_STATIC_CALL(pv_vcpu_is_preempted, dummy_vcpu_is_preempted);
+
 static bool steal_acc = true;
 static int __init parse_no_stealacc(char *arg)
 {
@@ -165,3 +170,92 @@ int __init pv_time_init(void)
 
 	return 0;
 }
+
+bool dummy_vcpu_is_preempted(unsigned int cpu)
+{
+	return false;
+}
+
+static bool __vcpu_is_preempted(unsigned int cpu)
+{
+	struct vcpu_state *st;
+
+	st = &per_cpu(vcpus_states, cpu);
+	return READ_ONCE(st->preempted);
+}
+
+static bool has_pv_vcpu_state(void)
+{
+	struct arm_smccc_res res;
+
+	/* To detect the presence of PV time support we require SMCCC 1.1+ */
+	if (arm_smccc_1_1_get_conduit() == SMCCC_CONDUIT_NONE)
+		return false;
+
+	arm_smccc_1_1_invoke(ARM_SMCCC_ARCH_FEATURES_FUNC_ID,
+			     ARM_SMCCC_HV_PV_VCPU_STATE_FEATURES,
+			     &res);
+
+	if (res.a0 != SMCCC_RET_SUCCESS)
+		return false;
+	return true;
+}
+
+static int __pv_vcpu_state_hook(unsigned int cpu, int event)
+{
+	struct arm_smccc_res res;
+	struct vcpu_state *st;
+
+	st = &per_cpu(vcpus_states, cpu);
+	arm_smccc_1_1_invoke(event, virt_to_phys(st), &res);
+	if (res.a0 != SMCCC_RET_SUCCESS)
+		return -EINVAL;
+	return 0;
+}
+
+static int vcpu_state_init(unsigned int cpu)
+{
+	int ret = __pv_vcpu_state_hook(cpu, ARM_SMCCC_HV_PV_VCPU_STATE_INIT);
+
+	if (ret)
+		pr_warn("Unable to ARM_SMCCC_HV_PV_STATE_INIT\n");
+	return ret;
+}
+
+static int vcpu_state_release(unsigned int cpu)
+{
+	int ret = __pv_vcpu_state_hook(cpu, ARM_SMCCC_HV_PV_VCPU_STATE_RELEASE);
+
+	if (ret)
+		pr_warn("Unable to ARM_SMCCC_HV_PV_STATE_RELEASE\n");
+	return ret;
+}
+
+static int pv_vcpu_state_register_hooks(void)
+{
+	int ret;
+
+	ret = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN,
+				"hypervisor/arm/pvstate:starting",
+				vcpu_state_init,
+				vcpu_state_release);
+	if (ret < 0)
+		pr_warn("Failed to register CPU hooks\n");
+	return 0;
+}
+
+int __init pv_vcpu_state_init(void)
+{
+	int ret;
+
+	if (!has_pv_vcpu_state())
+		return 0;
+
+	ret = pv_vcpu_state_register_hooks();
+	if (ret)
+		return ret;
+
+	static_call_update(pv_vcpu_is_preempted, __vcpu_is_preempted);
+	static_key_slow_inc(&pv_vcpu_is_preempted_enabled);
+	return 0;
+}
diff --git a/arch/arm64/kernel/smp.c b/arch/arm64/kernel/smp.c
index 6f6ff072acbd..20d42e0f2a99 100644
--- a/arch/arm64/kernel/smp.c
+++ b/arch/arm64/kernel/smp.c
@@ -50,6 +50,7 @@
 #include <asm/tlbflush.h>
 #include <asm/ptrace.h>
 #include <asm/virt.h>
+#include <asm/paravirt.h>
 
 #define CREATE_TRACE_POINTS
 #include <trace/events/ipi.h>
@@ -756,6 +757,9 @@ void __init smp_prepare_cpus(unsigned int max_cpus)
 	numa_store_cpu_info(this_cpu);
 	numa_add_cpu(this_cpu);
 
+	/* Init paravirt CPU state */
+	pv_vcpu_state_init();
+
 	/*
 	 * If UP is mandated by "nosmp" (which implies "maxcpus=0"), don't set
 	 * secondary CPUs present.
-- 
2.32.0.93.g670b81a890-goog