[RFC 1/2] riscv: process: Introduce idle thread using Zawrs extension

From: Xu Lu
Date: Thu Apr 18 2024 - 07:51:05 EST

The Zawrs extension introduces a new instruction WRS.NTO, which will
register a reservation set and causes the hart to temporarily stall
execution in a low-power state until a store occurs to the reservation
set or an interrupt is observed.

This commit implements new version of idle thread for RISC-V via Zawrs
extension.

Signed-off-by: Xu Lu <luxu.kernel@xxxxxxxxxxxxx>
Reviewed-by: Hangjing Li <lihangjing@xxxxxxxxxxxxx>
Reviewed-by: Liang Deng <dengliang.1214@xxxxxxxxxxxxx>
Reviewed-by: Wen Chai <chaiwen.cc@xxxxxxxxxxxxx>
---
arch/riscv/Kconfig | 24 +++++++++++++++++
arch/riscv/include/asm/cpuidle.h | 11 +-------
arch/riscv/include/asm/hwcap.h | 1 +
arch/riscv/include/asm/processor.h | 17 +++++++++++++
arch/riscv/kernel/cpu.c | 5 ++++
arch/riscv/kernel/cpufeature.c | 1 +
arch/riscv/kernel/process.c | 41 +++++++++++++++++++++++++++++-
7 files changed, 89 insertions(+), 11 deletions(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index be09c8836d56..a0d344e9803f 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -19,6 +19,7 @@ config RISCV
select ARCH_ENABLE_SPLIT_PMD_PTLOCK if PGTABLE_LEVELS > 2
select ARCH_ENABLE_THP_MIGRATION if TRANSPARENT_HUGEPAGE
select ARCH_HAS_BINFMT_FLAT
+ select ARCH_HAS_CPU_FINALIZE_INIT
select ARCH_HAS_CURRENT_STACK_POINTER
select ARCH_HAS_DEBUG_VIRTUAL if MMU
select ARCH_HAS_DEBUG_VM_PGTABLE
@@ -525,6 +526,20 @@ config RISCV_ISA_SVPBMT

If you don't know what to do here, say Y.

+config RISCV_ISA_ZAWRS
+ bool "Zawrs extension support for wait-on-reservation-set instructions"
+ depends on RISCV_ALTERNATIVE
+ default y
+ help
+ Adds support to dynamically detect the presence of the Zawrs
+ extension and enable its usage.
+
+ The Zawrs extension defines a pair of instructions to be used
+ in polling loops that allows a core to enter a low-power state
+ and wait on a store to a memory location.
+
+ If you don't know what to do here, say Y.
+
config TOOLCHAIN_HAS_V
bool
default y
@@ -1075,6 +1090,15 @@ endmenu # "Power management options"

menu "CPU Power Management"

+config RISCV_ZAWRS_IDLE
+ bool "Idle thread using ZAWRS extensions"
+ depends on RISCV_ISA_ZAWRS
+ default y
+ help
+ Adds support to implement idle thread using ZAWRS extension.
+
+ If you don't know what to do here, say Y.
+
source "drivers/cpuidle/Kconfig"

source "drivers/cpufreq/Kconfig"
diff --git a/arch/riscv/include/asm/cpuidle.h b/arch/riscv/include/asm/cpuidle.h
index 71fdc607d4bc..94c9ecb46571 100644
--- a/arch/riscv/include/asm/cpuidle.h
+++ b/arch/riscv/include/asm/cpuidle.h
@@ -10,15 +10,6 @@
#include <asm/barrier.h>
#include <asm/processor.h>

-static inline void cpu_do_idle(void)
-{
- /*
- * Add mb() here to ensure that all
- * IO/MEM accesses are completed prior
- * to entering WFI.
- */
- mb();
- wait_for_interrupt();
-}
+void cpu_do_idle(void);

#endif
diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
index e17d0078a651..5b358c3cf212 100644
--- a/arch/riscv/include/asm/hwcap.h
+++ b/arch/riscv/include/asm/hwcap.h
@@ -81,6 +81,7 @@
#define RISCV_ISA_EXT_ZTSO 72
#define RISCV_ISA_EXT_ZACAS 73
#define RISCV_ISA_EXT_XANDESPMU 74
+#define RISCV_ISA_EXT_ZAWRS 75

#define RISCV_ISA_EXT_XLINUXENVCFG 127

diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
index 0faf5f161f1e..1143367de8c6 100644
--- a/arch/riscv/include/asm/processor.h
+++ b/arch/riscv/include/asm/processor.h
@@ -157,6 +157,21 @@ static inline void wait_for_interrupt(void)
__asm__ __volatile__ ("wfi");
}

+static inline void wrs_nto(unsigned long *addr)
+{
+ int val;
+
+ __asm__ __volatile__(
+#ifdef CONFIG_64BIT
+ "lr.d %[p], %[v] \n\t"
+#else
+ "lr.w %[p], %[v] \n\t"
+#endif
+ ".long 0x00d00073 \n\t"
+ : [p] "=&r" (val), [v] "+A" (*addr)
+ : : "memory");
+}
+
extern phys_addr_t dma32_phys_limit;

struct device_node;
@@ -183,6 +198,8 @@ extern int set_unalign_ctl(struct task_struct *tsk, unsigned int val);
#define GET_UNALIGN_CTL(tsk, addr) get_unalign_ctl((tsk), (addr))
#define SET_UNALIGN_CTL(tsk, val) set_unalign_ctl((tsk), (val))

+extern void select_idle_routine(void);
+
#endif /* __ASSEMBLY__ */

#endif /* _ASM_RISCV_PROCESSOR_H */
diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
index d11d6320fb0d..69cebd41f5f3 100644
--- a/arch/riscv/kernel/cpu.c
+++ b/arch/riscv/kernel/cpu.c
@@ -22,6 +22,11 @@ bool arch_match_cpu_phys_id(int cpu, u64 phys_id)
return phys_id == cpuid_to_hartid_map(cpu);
}

+void __init arch_cpu_finalize_init(void)
+{
+ select_idle_routine();
+}
+
/*
* Returns the hart ID of the given device tree node, or -ENODEV if the node
* isn't an enabled and valid RISC-V hart node.
diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
index 3ed2359eae35..c080e6ca54ba 100644
--- a/arch/riscv/kernel/cpufeature.c
+++ b/arch/riscv/kernel/cpufeature.c
@@ -305,6 +305,7 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = {
__RISCV_ISA_EXT_DATA(svnapot, RISCV_ISA_EXT_SVNAPOT),
__RISCV_ISA_EXT_DATA(svpbmt, RISCV_ISA_EXT_SVPBMT),
__RISCV_ISA_EXT_DATA(xandespmu, RISCV_ISA_EXT_XANDESPMU),
+ __RISCV_ISA_EXT_DATA(zawrs, RISCV_ISA_EXT_ZAWRS),
};

const size_t riscv_isa_ext_count = ARRAY_SIZE(riscv_isa_ext);
diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
index 92922dbd5b5c..9f0f7b888bc1 100644
--- a/arch/riscv/kernel/process.c
+++ b/arch/riscv/kernel/process.c
@@ -15,6 +15,7 @@
#include <linux/tick.h>
#include <linux/ptrace.h>
#include <linux/uaccess.h>
+#include <linux/static_call.h>

#include <asm/unistd.h>
#include <asm/processor.h>
@@ -37,11 +38,49 @@ EXPORT_SYMBOL(__stack_chk_guard);

extern asmlinkage void ret_from_fork(void);

-void arch_cpu_idle(void)
+static __cpuidle void default_idle(void)
+{
+ /*
+ * Add mb() here to ensure that all
+ * IO/MEM accesses are completed prior
+ * to entering WFI.
+ */
+ mb();
+ wait_for_interrupt();
+}
+
+static __cpuidle void wrs_idle(void)
+{
+ /*
+ * Add mb() here to ensure that all
+ * IO/MEM accesses are completed prior
+ * to entering WRS.NTO.
+ */
+ mb();
+ wrs_nto(&current_thread_info()->flags);
+}
+
+DEFINE_STATIC_CALL_NULL(riscv_idle, default_idle);
+
+void __cpuidle cpu_do_idle(void)
+{
+ static_call(riscv_idle)();
+}
+
+void __cpuidle arch_cpu_idle(void)
{
cpu_do_idle();
}

+void __init select_idle_routine(void)
+{
+ if (IS_ENABLED(CONFIG_RISCV_ZAWRS_IDLE) &&
+ riscv_has_extension_likely(RISCV_ISA_EXT_ZAWRS))
+ static_call_update(riscv_idle, wrs_idle);
+ else
+ static_call_update(riscv_idle, default_idle);
+}
+
int set_unalign_ctl(struct task_struct *tsk, unsigned int val)
{
if (!unaligned_ctl_available())
--
2.20.1