[RFC][PATCH 2/4] arm64: add guest pvstate support

[Sergey Senozhatsky <sergey.senozhatsky@gmail.com>]

PV-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 SMCC HV call, so that host can update vCPU state
when it puts or loads vCPU.

This has impact on the guest's scheduler - it does check
the state of the vCPU it wants to run a task on:

[..]
  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 <sergey.senozhatsky@gmail.com>
---
 arch/arm64/include/asm/paravirt.h  |  15 ++++
 arch/arm64/kernel/Makefile         |   2 +-
 arch/arm64/kernel/paravirt-state.c | 117 +++++++++++++++++++++++++++++
 arch/arm64/kernel/paravirt.c       |   4 +-
 arch/arm64/kernel/time.c           |   1 +
 5 files changed, 137 insertions(+), 2 deletions(-)
 create mode 100644 arch/arm64/kernel/paravirt-state.c

diff --git a/arch/arm64/include/asm/paravirt.h b/arch/arm64/include/asm/paravirt.h
index cf3a0fd7c1a7..1bf164b2041b 100644
--- a/arch/arm64/include/asm/paravirt.h
+++ b/arch/arm64/include/asm/paravirt.h
@@ -7,12 +7,22 @@ struct static_key;
 extern struct static_key paravirt_steal_enabled;
 extern struct static_key paravirt_steal_rq_enabled;
 
+struct pvstate_vcpu_info {
+	bool	preempted;
+	u8	reserved[63];
+};
+
+struct pv_state_ops {
+	bool (*vcpu_is_preempted)(int cpu);
+};
+
 struct pv_time_ops {
 	unsigned long long (*steal_clock)(int cpu);
 };
 
 struct paravirt_patch_template {
 	struct pv_time_ops time;
+	struct pv_state_ops state;
 };
 
 extern struct paravirt_patch_template pv_ops;
@@ -22,10 +32,15 @@ static inline u64 paravirt_steal_clock(int cpu)
 	return pv_ops.time.steal_clock(cpu);
 }
 
+bool native_vcpu_is_preempted(int cpu);
+bool paravirt_vcpu_is_preempted(int cpu);
+
+int __init pv_state_init(void);
 int __init pv_time_init(void);
 
 #else
 
+#define pv_state_init() do {} while (0)
 #define pv_time_init() do {} while (0)
 
 #endif // CONFIG_PARAVIRT
diff --git a/arch/arm64/kernel/Makefile b/arch/arm64/kernel/Makefile
index 5fb9b728459b..18974d5e798d 100644
--- a/arch/arm64/kernel/Makefile
+++ b/arch/arm64/kernel/Makefile
@@ -48,7 +48,7 @@ obj-$(CONFIG_ARMV8_DEPRECATED)		+= armv8_deprecated.o
 obj-$(CONFIG_ACPI)			+= acpi.o
 obj-$(CONFIG_ACPI_NUMA)			+= acpi_numa.o
 obj-$(CONFIG_ARM64_ACPI_PARKING_PROTOCOL)	+= acpi_parking_protocol.o
-obj-$(CONFIG_PARAVIRT)			+= paravirt.o
+obj-$(CONFIG_PARAVIRT)			+= paravirt.o paravirt-state.o
 obj-$(CONFIG_RANDOMIZE_BASE)		+= kaslr.o
 obj-$(CONFIG_HIBERNATION)		+= hibernate.o hibernate-asm.o
 obj-$(CONFIG_KEXEC_CORE)		+= machine_kexec.o relocate_kernel.o	\
diff --git a/arch/arm64/kernel/paravirt-state.c b/arch/arm64/kernel/paravirt-state.c
new file mode 100644
index 000000000000..4ae92a84c73d
--- /dev/null
+++ b/arch/arm64/kernel/paravirt-state.c
@@ -0,0 +1,117 @@
+// SPDX-License-Identifier: GPL-2.0-only
+
+#define pr_fmt(fmt) "arm-pvstate: " fmt
+
+#include <linux/arm-smccc.h>
+#include <linux/cpuhotplug.h>
+#include <linux/export.h>
+#include <linux/io.h>
+#include <linux/jump_label.h>
+#include <linux/printk.h>
+#include <linux/psci.h>
+#include <linux/reboot.h>
+#include <linux/slab.h>
+#include <linux/types.h>
+
+#include <asm/paravirt.h>
+#include <asm/smp_plat.h>
+
+static DEFINE_PER_CPU(struct pvstate_vcpu_info, vcpus_states);
+
+bool native_vcpu_is_preempted(int cpu)
+{
+	return false;
+}
+
+static bool pv_vcpu_is_preempted(int cpu)
+{
+	struct pvstate_vcpu_info *st;
+
+	st = &per_cpu(vcpus_states, cpu);
+	return READ_ONCE(st->preempted);
+}
+
+bool paravirt_vcpu_is_preempted(int cpu)
+{
+	return pv_ops.state.vcpu_is_preempted(cpu);
+}
+
+static bool has_pvstate(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_STATE_FEATURES,
+			     &res);
+
+	if (res.a0 != SMCCC_RET_SUCCESS)
+		return false;
+	return true;
+}
+
+static int __pvstate_cpu_hook(unsigned int cpu, int event)
+{
+	struct arm_smccc_res res;
+	struct pvstate_vcpu_info *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 pvstate_cpu_init(unsigned int cpu)
+{
+	int ret = __pvstate_cpu_hook(cpu, ARM_SMCCC_HV_PV_STATE_INIT);
+
+	if (ret)
+		pr_warn("Unable to ARM_SMCCC_HV_PV_STATE_INIT\n");
+	return ret;
+}
+
+static int pvstate_cpu_release(unsigned int cpu)
+{
+	int ret = __pvstate_cpu_hook(cpu, ARM_SMCCC_HV_PV_STATE_RELEASE);
+
+	if (ret)
+		pr_warn("Unable to ARM_SMCCC_HV_PV_STATE_RELEASE\n");
+	return ret;
+}
+
+static int pvstate_register_hooks(void)
+{
+	int ret;
+
+	ret = cpuhp_setup_state(CPUHP_AP_ARM_KVMPV_STARTING,
+			"hypervisor/arm/pvstate:starting",
+			pvstate_cpu_init,
+			pvstate_cpu_release);
+	if (ret < 0)
+		pr_warn("Failed to register CPU hooks\n");
+	return ret;
+}
+
+static int __pvstate_init(void)
+{
+	return pvstate_register_hooks();
+}
+
+int __init pv_state_init(void)
+{
+	int ret;
+
+	if (!has_pvstate())
+		return 0;
+
+	ret = __pvstate_init();
+	if (ret)
+		return ret;
+
+	pv_ops.state.vcpu_is_preempted = pv_vcpu_is_preempted;
+	return 0;
+}
diff --git a/arch/arm64/kernel/paravirt.c b/arch/arm64/kernel/paravirt.c
index 295d66490584..3fec7563ac27 100644
--- a/arch/arm64/kernel/paravirt.c
+++ b/arch/arm64/kernel/paravirt.c
@@ -26,7 +26,9 @@
 struct static_key paravirt_steal_enabled;
 struct static_key paravirt_steal_rq_enabled;
 
-struct paravirt_patch_template pv_ops;
+struct paravirt_patch_template pv_ops = {
+	.state.vcpu_is_preempted = native_vcpu_is_preempted,
+};
 EXPORT_SYMBOL_GPL(pv_ops);
 
 struct pv_time_stolen_time_region {
diff --git a/arch/arm64/kernel/time.c b/arch/arm64/kernel/time.c
index eebbc8d7123e..50c55792f72b 100644
--- a/arch/arm64/kernel/time.c
+++ b/arch/arm64/kernel/time.c
@@ -68,4 +68,5 @@ void __init time_init(void)
 	lpj_fine = arch_timer_rate / HZ;
 
 	pv_time_init();
+	pv_state_init();
 }
-- 
2.27.0