[PATCH RFC v4 2/4] RISC-V: paravirt: Add pvqspinlock frontend
GUO Ren (XuanTie)
guoren at kernel.org
Mon Sep 21 05:27:02 PDT 2026
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 at redhat.com>
Signed-off-by: GUO Ren (XuanTie) <guoren at 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 at 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 at 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 at 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
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