[PATCH RFC v4 2/4] RISC-V: paravirt: Add pvqspinlock frontend

["GUO Ren (XuanTie)" <guoren@kernel.org>]

Add a virtualization-friendly unfair qspinlock frontend that halts the
virtual CPU instead of spinning.

Use static_call to switch between:
  native_queued_spin_lock_slowpath()    __pv_queued_spin_lock_slowpath()
  native_queued_spin_unlock()           __pv_queued_spin_unlock()

Add the pv_wait and pv_kick implementations.

Reviewed-by: Leonardo Bras <leobras@redhat.com>
Signed-off-by: GUO Ren (XuanTie) <guoren@kernel.org>
---
 arch/riscv/Kconfig                          | 12 ++++++
 arch/riscv/include/asm/Kbuild               |  1 -
 arch/riscv/include/asm/qspinlock.h          | 33 ++++++++++++++
 arch/riscv/include/asm/qspinlock_paravirt.h | 26 +++++++++++
 arch/riscv/kernel/Makefile                  |  2 +
 arch/riscv/kernel/qspinlock_paravirt.c      | 67 +++++++++++++++++++++++++++++
 arch/riscv/kernel/setup.c                   |  5 +++
 7 files changed, 145 insertions(+), 1 deletion(-)

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index d6c2dbf8455c..9af53ef5ee95 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -1165,6 +1165,18 @@ config PARAVIRT_TIME_ACCOUNTING
 
 	  If in doubt, say N here.
 
+config PARAVIRT_SPINLOCKS
+	bool "Paravirtualization layer for spinlocks"
+	depends on QUEUED_SPINLOCKS
+	default y
+	help
+	  Paravirtualized spinlocks allow a unfair qspinlock to replace the
+	  test-set kvm-guest virt spinlock implementation with something
+	  virtualization-friendly, for example, halt the virtual CPU rather
+	  than spinning.
+
+	  If you are unsure how to answer this question, answer Y.
+
 config RELOCATABLE
 	bool "Build a relocatable kernel"
 	select MODULE_SECTIONS if MODULES
diff --git a/arch/riscv/include/asm/Kbuild b/arch/riscv/include/asm/Kbuild
index 7721b63642f4..3d8800d4fe48 100644
--- a/arch/riscv/include/asm/Kbuild
+++ b/arch/riscv/include/asm/Kbuild
@@ -13,7 +13,6 @@ generic-y += spinlock_types.h
 generic-y += ticket_spinlock.h
 generic-y += qrwlock.h
 generic-y += qrwlock_types.h
-generic-y += qspinlock.h
 generic-y += ring_buffer.h
 generic-y += user.h
 generic-y += vmlinux.lds.h
diff --git a/arch/riscv/include/asm/qspinlock.h b/arch/riscv/include/asm/qspinlock.h
new file mode 100644
index 000000000000..330b714edc44
--- /dev/null
+++ b/arch/riscv/include/asm/qspinlock.h
@@ -0,0 +1,33 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2026 GUO Ren (XuanTie) <guoren@kernel.org>
+ */
+
+#ifndef _ASM_RISCV_QSPINLOCK_H
+#define _ASM_RISCV_QSPINLOCK_H
+
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+#include <asm/qspinlock_paravirt.h>
+
+/* How long a lock should spin before we consider blocking */
+#define SPIN_THRESHOLD		(1 << 15)
+
+void native_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
+void __pv_init_lock_hash(void);
+void __pv_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
+
+static inline void queued_spin_lock_slowpath(struct qspinlock *lock, u32 val)
+{
+	static_call(pv_queued_spin_lock_slowpath)(lock, val);
+}
+
+#define queued_spin_unlock	queued_spin_unlock
+static inline void queued_spin_unlock(struct qspinlock *lock)
+{
+	static_call(pv_queued_spin_unlock)(lock);
+}
+#endif /* CONFIG_PARAVIRT_SPINLOCKS */
+
+#include <asm-generic/qspinlock.h>
+
+#endif /* _ASM_RISCV_QSPINLOCK_H */
diff --git a/arch/riscv/include/asm/qspinlock_paravirt.h b/arch/riscv/include/asm/qspinlock_paravirt.h
new file mode 100644
index 000000000000..7261cd9b13b2
--- /dev/null
+++ b/arch/riscv/include/asm/qspinlock_paravirt.h
@@ -0,0 +1,26 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2026 GUO Ren (XuanTie) <guoren@kernel.org>
+ */
+
+#ifndef _ASM_RISCV_QSPINLOCK_PARAVIRT_H
+#define _ASM_RISCV_QSPINLOCK_PARAVIRT_H
+
+void pv_wait(u8 *ptr, u8 val);
+void pv_kick(int cpu);
+
+void dummy_queued_spin_lock_slowpath(struct qspinlock *lock, u32 val);
+void dummy_queued_spin_unlock(struct qspinlock *lock);
+
+DECLARE_STATIC_CALL(pv_queued_spin_lock_slowpath, dummy_queued_spin_lock_slowpath);
+DECLARE_STATIC_CALL(pv_queued_spin_unlock, dummy_queued_spin_unlock);
+
+bool __init pv_qspinlock_init(void);
+
+void __pv_queued_spin_unlock_slowpath(struct qspinlock *lock, u8 locked);
+
+bool pv_is_native_spin_unlock(void);
+
+void __pv_queued_spin_unlock(struct qspinlock *lock);
+
+#endif /* _ASM_RISCV_QSPINLOCK_PARAVIRT_H */
diff --git a/arch/riscv/kernel/Makefile b/arch/riscv/kernel/Makefile
index ebe1c3588177..b9b0763dba32 100644
--- a/arch/riscv/kernel/Makefile
+++ b/arch/riscv/kernel/Makefile
@@ -130,3 +130,5 @@ obj-$(CONFIG_GENERIC_CPU_VULNERABILITIES) += bugs.o
 obj-$(CONFIG_RISCV_USER_CFI) += usercfi.o
 
 obj-$(CONFIG_RISCV_ISA_SSQOSID) += qos.o
+
+obj-$(CONFIG_PARAVIRT_SPINLOCKS) += qspinlock_paravirt.o
diff --git a/arch/riscv/kernel/qspinlock_paravirt.c b/arch/riscv/kernel/qspinlock_paravirt.c
new file mode 100644
index 000000000000..c534447437fe
--- /dev/null
+++ b/arch/riscv/kernel/qspinlock_paravirt.c
@@ -0,0 +1,67 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2026 GUO Ren (XuanTie) <guoren@kernel.org>
+ */
+
+#include <linux/static_call.h>
+#include <asm/qspinlock_paravirt.h>
+#include <asm/sbi.h>
+
+void pv_kick(int cpu)
+{
+	sbi_ecall(SBI_EXT_PVLOCK, SBI_EXT_PVLOCK_KICK_CPU,
+		  cpuid_to_hartid_map(cpu), 0, 0, 0, 0, 0);
+}
+
+void pv_wait(u8 *ptr, u8 val)
+{
+	unsigned long flags;
+
+	if (in_nmi())
+		return;
+
+	local_irq_save(flags);
+	if (READ_ONCE(*ptr) != val)
+		goto out;
+
+	wait_for_interrupt();
+out:
+	local_irq_restore(flags);
+}
+
+static void native_queued_spin_unlock(struct qspinlock *lock)
+{
+	/*
+	 * unlock() needs release semantics:
+	 */
+	smp_store_release(&lock->locked, 0);
+}
+
+DEFINE_STATIC_CALL(pv_queued_spin_lock_slowpath, native_queued_spin_lock_slowpath);
+EXPORT_STATIC_CALL(pv_queued_spin_lock_slowpath);
+
+DEFINE_STATIC_CALL(pv_queued_spin_unlock, native_queued_spin_unlock);
+EXPORT_STATIC_CALL(pv_queued_spin_unlock);
+
+bool __init pv_qspinlock_init(void)
+{
+	if (num_possible_cpus() == 1)
+		return false;
+
+	if (!sbi_probe_extension(SBI_EXT_PVLOCK))
+		return false;
+
+	pr_info("PV qspinlocks enabled\n");
+	__pv_init_lock_hash();
+
+	static_call_update(pv_queued_spin_lock_slowpath, __pv_queued_spin_lock_slowpath);
+	static_call_update(pv_queued_spin_unlock, __pv_queued_spin_unlock);
+
+	return true;
+}
+
+bool pv_is_native_spin_unlock(void)
+{
+	return static_call_query(pv_queued_spin_unlock) ==
+	       native_queued_spin_unlock;
+}
diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
index a32344bb220d..ccfa92b7c81a 100644
--- a/arch/riscv/kernel/setup.c
+++ b/arch/riscv/kernel/setup.c
@@ -281,6 +281,11 @@ static void __init riscv_spinlock_init(void)
 		return;
 	}
 
+#ifdef CONFIG_PARAVIRT_SPINLOCKS
+	if (pv_qspinlock_init())
+		return;
+#endif
+
 	if (IS_ENABLED(CONFIG_RISCV_ISA_ZABHA) &&
 	    IS_ENABLED(CONFIG_RISCV_ISA_ZACAS) &&
 	    IS_ENABLED(CONFIG_TOOLCHAIN_HAS_ZACAS) &&

-- 
2.43.0