[PATCH RFC v4 0/4] RISC-V: Add PARAVIRT_SPINLOCKS support

[PATCH RFC v4 0/4] RISC-V: Add PARAVIRT_SPINLOCKS support

[GUO Ren (XuanTie)]

Paravirtualized spinlocks allow an unfair qspinlock to replace the
ticket-lock or native fair qspinlock implementation with something
virtualization-friendly, for example, halting the virtual CPU rather
than spinning.

You can observe the paravirt qspinlock behavior with the pv_kick and
pv_wait tracepoints under /sys/kernel/debug/tracing/:

  # echo 1 > /sys/kernel/debug/tracing/events/paravirt/enable
  # cat /sys/kernel/debug/tracing/trace
            TASK-PID     CPU#  |||||  TIMESTAMP  FUNCTION
               | |         |   |||||     |         |
             sh-100     [001] d..2.    28.312294: pv_wait: cpu 1 out of wfi
         <idle>-0       [000] d.h4.    28.322030: pv_kick: cpu 0 kick target cpu 1

This series is split from [1]. The newest discussion is at [2].

[1]: https://lore.kernel.org/linux-riscv/20231225125847.2778638-1-guoren@kernel.org/
[2]: https://lists.riscv.org/g/tech-prs/message/1211

Changelog:
v4:
 - Rebase onto v7.3-rc4.
 - Normalize the author identity to "GUO Ren (XuanTie)".
 - Minor commit log cleanups (fix line-wrap artifacts and typos).

v3:
https://lore.kernel.org/linux-riscv/20251201003041.695081-1-guoren@kernel.org/
 - Rebase on linux-6.18-rc7.
 - Simplify nopvspin usage.

v2:
https://lore.kernel.org/linux-riscv/20241227011011.2331381-1-guoren@kernel.org/
 - Add RFC tag.
 - Using new SBI_EXT_PVLOCK ID.
 - Add virt_spin_lock support.
 - Add nopvspin support.

v1:
https://lore.kernel.org/linux-riscv/20241222033917.1754495-1-guoren@kernel.org/

GUO Ren (XuanTie) (5):
  RISC-V: Add PARAVIRT_SPINLOCKS support
  RISC-V: paravirt: Add pvqspinlock KVM backend
  RISC-V: paravirt: Add pvqspinlock frontend
  RISC-V: paravirt: pvqspinlock: Add trace point for pv_kick/wait
  RISC-V: paravirt: Support nopvspin to disable PARAVIRT_SPINLOCKS

 .../admin-guide/kernel-parameters.txt         |  2 +-
 arch/riscv/Kconfig                            | 12 +++
 arch/riscv/include/asm/Kbuild                 |  1 -
 arch/riscv/include/asm/kvm_vcpu_sbi.h         |  1 +
 arch/riscv/include/asm/qspinlock.h            | 57 +++++++++++++
 arch/riscv/include/asm/qspinlock_paravirt.h   | 26 ++++++
 arch/riscv/include/asm/sbi.h                  |  5 ++
 arch/riscv/include/uapi/asm/kvm.h             |  1 +
 arch/riscv/kernel/Makefile                    |  2 +
 arch/riscv/kernel/qspinlock_paravirt.c        | 82 +++++++++++++++++++
 arch/riscv/kernel/setup.c                     |  5 ++
 .../kernel/trace_events_filter_paravirt.h     | 58 +++++++++++++
 arch/riscv/kvm/Makefile                       |  1 +
 arch/riscv/kvm/vcpu_sbi.c                     |  4 +
 arch/riscv/kvm/vcpu_sbi_pvlock.c              | 54 ++++++++++++
 15 files changed, 309 insertions(+), 2 deletions(-)
 create mode 100644 arch/riscv/include/asm/qspinlock.h
 create mode 100644 arch/riscv/include/asm/qspinlock_paravirt.h
 create mode 100644 arch/riscv/kernel/qspinlock_paravirt.c
 create mode 100644 arch/riscv/kernel/trace_events_filter_paravirt.h
 create mode 100644 arch/riscv/kvm/vcpu_sbi_pvlock.c

--
2.43.1

---
GUO Ren (XuanTie) (4):
      RISC-V: paravirt: Add pvqspinlock KVM backend
      RISC-V: paravirt: Add pvqspinlock frontend
      RISC-V: paravirt: pvqspinlock: Add trace point for pv_kick/wait
      RISC-V: paravirt: Support nopvspin to disable PARAVIRT_SPINLOCKS

 Documentation/admin-guide/kernel-parameters.txt  |  2 +-
 arch/riscv/Kconfig                               | 12 ++++
 arch/riscv/include/asm/Kbuild                    |  1 -
 arch/riscv/include/asm/kvm_vcpu_sbi.h            |  1 +
 arch/riscv/include/asm/qspinlock.h               | 57 ++++++++++++++++
 arch/riscv/include/asm/qspinlock_paravirt.h      | 26 ++++++++
 arch/riscv/include/asm/sbi.h                     |  5 ++
 arch/riscv/include/uapi/asm/kvm.h                |  1 +
 arch/riscv/kernel/Makefile                       |  2 +
 arch/riscv/kernel/qspinlock_paravirt.c           | 82 ++++++++++++++++++++++++
 arch/riscv/kernel/setup.c                        |  5 ++
 arch/riscv/kernel/trace_events_filter_paravirt.h | 58 +++++++++++++++++
 arch/riscv/kvm/Makefile                          |  1 +
 arch/riscv/kvm/vcpu_sbi.c                        |  4 ++
 arch/riscv/kvm/vcpu_sbi_pvlock.c                 | 54 ++++++++++++++++
 15 files changed, 309 insertions(+), 2 deletions(-)
---
base-commit: 93f51579e7df248780214094418f205253383cc5
change-id: 20260921-pvqspinlock-73ad0cfb45b1

Best regards,
--  
GUO Ren (XuanTie) <guoren@kernel.org>

[PATCH RFC v4 1/4] RISC-V: paravirt: Add pvqspinlock KVM backend

[GUO Ren (XuanTie)]

Add the KVM backend for the SBI PVLOCK (paravirt qspinlock kick_cpu)
extension. kvm_sbi_ext_pvlock_kick_cpu() kicks the target vCPU out of
the halted state via kvm_vcpu_kick(), and yields to it with
kvm_vcpu_yield_to() when the target is already runnable.

Reviewed-by: Leonardo Bras <leobras@redhat.com>
Signed-off-by: GUO Ren (XuanTie) <guoren@kernel.org>
---
 arch/riscv/include/asm/kvm_vcpu_sbi.h |  1 +
 arch/riscv/include/asm/sbi.h          |  5 ++++
 arch/riscv/include/uapi/asm/kvm.h     |  1 +
 arch/riscv/kvm/Makefile               |  1 +
 arch/riscv/kvm/vcpu_sbi.c             |  4 +++
 arch/riscv/kvm/vcpu_sbi_pvlock.c      | 54 +++++++++++++++++++++++++++++++++++
 6 files changed, 66 insertions(+)

diff --git a/arch/riscv/include/asm/kvm_vcpu_sbi.h b/arch/riscv/include/asm/kvm_vcpu_sbi.h
index f01a2860c751..cdc16e64ad19 100644
--- a/arch/riscv/include/asm/kvm_vcpu_sbi.h
+++ b/arch/riscv/include/asm/kvm_vcpu_sbi.h
@@ -114,6 +114,7 @@ extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_fwft;
 extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_mpxy;
 extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_experimental;
 extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_vendor;
+extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_pvlock;
 
 #ifdef CONFIG_RISCV_PMU_SBI
 extern const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_pmu;
diff --git a/arch/riscv/include/asm/sbi.h b/arch/riscv/include/asm/sbi.h
index 5725e0ca4dda..724ab834ad85 100644
--- a/arch/riscv/include/asm/sbi.h
+++ b/arch/riscv/include/asm/sbi.h
@@ -38,6 +38,7 @@ enum sbi_ext_id {
 	SBI_EXT_FWFT = 0x46574654,
 	SBI_EXT_MPXY = 0x4D505859,
 	SBI_EXT_DBTR = 0x44425452,
+	SBI_EXT_PVLOCK = 0x50564C4B,
 
 	/* Experimentals extensions must lie within this range */
 	SBI_EXT_EXPERIMENTAL_START = 0x08000000,
@@ -534,6 +535,10 @@ union sbi_dbtr_shmem_entry {
 	struct sbi_dbtr_id_msg id;
 };
 
+enum sbi_ext_pvlock_fid {
+	SBI_EXT_PVLOCK_KICK_CPU = 0,
+};
+
 /* SBI spec version fields */
 #define SBI_SPEC_VERSION_DEFAULT	0x1
 #define SBI_SPEC_VERSION_MAJOR_SHIFT	24
diff --git a/arch/riscv/include/uapi/asm/kvm.h b/arch/riscv/include/uapi/asm/kvm.h
index 20d9959ca44f..110a0d62113f 100644
--- a/arch/riscv/include/uapi/asm/kvm.h
+++ b/arch/riscv/include/uapi/asm/kvm.h
@@ -228,6 +228,7 @@ enum KVM_RISCV_SBI_EXT_ID {
 	KVM_RISCV_SBI_EXT_SUSP,
 	KVM_RISCV_SBI_EXT_FWFT,
 	KVM_RISCV_SBI_EXT_MPXY,
+	KVM_RISCV_SBI_EXT_PVLOCK,
 	KVM_RISCV_SBI_EXT_MAX,
 };
 
diff --git a/arch/riscv/kvm/Makefile b/arch/riscv/kvm/Makefile
index 296c2ba05089..404a4c5f8984 100644
--- a/arch/riscv/kvm/Makefile
+++ b/arch/riscv/kvm/Makefile
@@ -38,6 +38,7 @@ kvm-y += vcpu_sbi_sta.o
 kvm-y += vcpu_sbi_system.o
 kvm-$(CONFIG_RISCV_SBI_V01) += vcpu_sbi_v01.o
 kvm-y += vcpu_switch.o
+kvm-y += vcpu_sbi_pvlock.o
 kvm-y += vcpu_timer.o
 kvm-y += vcpu_vector.o
 kvm-y += vm.o
diff --git a/arch/riscv/kvm/vcpu_sbi.c b/arch/riscv/kvm/vcpu_sbi.c
index 1c4f874368fb..07f72e619fd5 100644
--- a/arch/riscv/kvm/vcpu_sbi.c
+++ b/arch/riscv/kvm/vcpu_sbi.c
@@ -94,6 +94,10 @@ static const struct kvm_riscv_sbi_extension_entry sbi_ext[] = {
 		.ext_idx = KVM_RISCV_SBI_EXT_VENDOR,
 		.ext_ptr = &vcpu_sbi_ext_vendor,
 	},
+	{
+		.ext_idx = KVM_RISCV_SBI_EXT_PVLOCK,
+		.ext_ptr = &vcpu_sbi_ext_pvlock,
+	},
 };
 
 static const struct kvm_riscv_sbi_extension_entry *
diff --git a/arch/riscv/kvm/vcpu_sbi_pvlock.c b/arch/riscv/kvm/vcpu_sbi_pvlock.c
new file mode 100644
index 000000000000..728e59f3f18f
--- /dev/null
+++ b/arch/riscv/kvm/vcpu_sbi_pvlock.c
@@ -0,0 +1,54 @@
+// SPDX-License-Identifier: GPL-2.0-only
+/*
+ * Copyright (C) 2026 GUO Ren (XuanTie) <guoren@kernel.org>
+ */
+
+#include <linux/errno.h>
+#include <linux/err.h>
+#include <linux/kvm_host.h>
+#include <asm/sbi.h>
+#include <asm/kvm_vcpu_sbi.h>
+
+static int kvm_sbi_ext_pvlock_kick_cpu(struct kvm_vcpu *vcpu)
+{
+	struct kvm_cpu_context *cp = &vcpu->arch.guest_context;
+	struct kvm *kvm = vcpu->kvm;
+	struct kvm_vcpu *target;
+
+	target = kvm_get_vcpu_by_id(kvm, cp->a0);
+	if (!target)
+		return SBI_ERR_INVALID_PARAM;
+
+	kvm_vcpu_kick(target);
+
+	if (READ_ONCE(target->ready))
+		kvm_vcpu_yield_to(target);
+
+	return SBI_SUCCESS;
+}
+
+static int kvm_sbi_ext_pvlock_handler(struct kvm_vcpu *vcpu, struct kvm_run *run,
+				      struct kvm_vcpu_sbi_return *retdata)
+{
+	int ret = 0;
+	struct kvm_cpu_context *cp = &vcpu->arch.guest_context;
+	unsigned long funcid = cp->a6;
+
+	switch (funcid) {
+	case SBI_EXT_PVLOCK_KICK_CPU:
+		ret = kvm_sbi_ext_pvlock_kick_cpu(vcpu);
+		break;
+	default:
+		ret = SBI_ERR_NOT_SUPPORTED;
+	}
+
+	retdata->err_val = ret;
+
+	return 0;
+}
+
+const struct kvm_vcpu_sbi_extension vcpu_sbi_ext_pvlock = {
+	.extid_start = SBI_EXT_PVLOCK,
+	.extid_end = SBI_EXT_PVLOCK,
+	.handler = kvm_sbi_ext_pvlock_handler,
+};

-- 
2.43.0

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

[GUO Ren (XuanTie)]

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

[PATCH RFC v4 3/4] RISC-V: paravirt: pvqspinlock: Add trace point for pv_kick/wait

[GUO Ren (XuanTie)]

Add tracepoints for pv_kick and pv_wait to observe the paravirt
qspinlock behavior:

  # cd /sys/kernel/debug/tracing
  # echo 1 > events/paravirt/enable
  # cat trace
  TASK-PID   CPU#  |||||  TIMESTAMP  FUNCTION
     | |       |   |||||     |         |
    sh-100   [001] d..2.  28.312294: pv_wait: cpu 1 out of wfi
  <idle>-0   [000] d.h4.  28.322030: pv_kick: cpu 0 kick target cpu 1

Reviewed-by: Leonardo Bras <leobras@redhat.com>
Signed-off-by: GUO Ren (XuanTie) <guoren@kernel.org>
---
 arch/riscv/kernel/qspinlock_paravirt.c           |  7 +++
 arch/riscv/kernel/trace_events_filter_paravirt.h | 58 ++++++++++++++++++++++++
 2 files changed, 65 insertions(+)

diff --git a/arch/riscv/kernel/qspinlock_paravirt.c b/arch/riscv/kernel/qspinlock_paravirt.c
index c534447437fe..04b13994e971 100644
--- a/arch/riscv/kernel/qspinlock_paravirt.c
+++ b/arch/riscv/kernel/qspinlock_paravirt.c
@@ -7,8 +7,13 @@
 #include <asm/qspinlock_paravirt.h>
 #include <asm/sbi.h>
 
+#define CREATE_TRACE_POINTS
+#include "trace_events_filter_paravirt.h"
+
 void pv_kick(int cpu)
 {
+	trace_pv_kick(smp_processor_id(), cpu);
+
 	sbi_ecall(SBI_EXT_PVLOCK, SBI_EXT_PVLOCK_KICK_CPU,
 		  cpuid_to_hartid_map(cpu), 0, 0, 0, 0, 0);
 }
@@ -25,6 +30,8 @@ void pv_wait(u8 *ptr, u8 val)
 		goto out;
 
 	wait_for_interrupt();
+
+	trace_pv_wait(smp_processor_id());
 out:
 	local_irq_restore(flags);
 }
diff --git a/arch/riscv/kernel/trace_events_filter_paravirt.h b/arch/riscv/kernel/trace_events_filter_paravirt.h
new file mode 100644
index 000000000000..8d57efda2c17
--- /dev/null
+++ b/arch/riscv/kernel/trace_events_filter_paravirt.h
@@ -0,0 +1,58 @@
+/* SPDX-License-Identifier: GPL-2.0-only */
+/*
+ * Copyright (C) 2026 GUO Ren (XuanTie) <guoren@kernel.org>
+ */
+#undef TRACE_SYSTEM
+#define TRACE_SYSTEM paravirt
+
+#if !defined(_TRACE_PARAVIRT_H) || defined(TRACE_HEADER_MULTI_READ)
+#define _TRACE_PARAVIRT_H
+
+#include <linux/tracepoint.h>
+
+TRACE_EVENT(pv_kick,
+	TP_PROTO(int cpu, int target),
+	TP_ARGS(cpu, target),
+
+	TP_STRUCT__entry(
+		__field(int, cpu)
+		__field(int, target)
+	),
+
+	TP_fast_assign(
+		__entry->cpu = cpu;
+		__entry->target = target;
+	),
+
+	TP_printk("cpu %d pv_kick target cpu %d",
+		__entry->cpu,
+		__entry->target
+	)
+);
+
+TRACE_EVENT(pv_wait,
+	TP_PROTO(int cpu),
+	TP_ARGS(cpu),
+
+	TP_STRUCT__entry(
+		__field(int, cpu)
+	),
+
+	TP_fast_assign(
+		__entry->cpu = cpu;
+	),
+
+	TP_printk("cpu %d out of wfi",
+		__entry->cpu
+	)
+);
+
+#endif /* _TRACE_PARAVIRT_H || TRACE_HEADER_MULTI_READ */
+
+#undef TRACE_INCLUDE_PATH
+#undef TRACE_INCLUDE_FILE
+#define TRACE_INCLUDE_PATH ../../../arch/riscv/kernel/
+#define TRACE_INCLUDE_FILE trace_events_filter_paravirt
+
+/* This part must be outside protection */
+#include <trace/define_trace.h>

-- 
2.43.0

[PATCH RFC v4 4/4] RISC-V: paravirt: Support nopvspin to disable PARAVIRT_SPINLOCKS

[GUO Ren (XuanTie)]

VM guests should fall back to a Test-and-Set spinlock when
PARAVIRT_SPINLOCKS is disabled, because fair locks suffer from severe
lock-holder preemption issues. The virt_spin_lock_key shortcuts
queued_spin_lock_slowpath(), allowing virt_spin_lock() to hijack it.
See commit 43b3f02899f7 ("locking/qspinlock/x86: Fix performance
regression under unaccelerated VMs").

Add a static key controlling whether virt_spin_lock() is called, and
add nopvspin support mirroring x86.

Signed-off-by: GUO Ren (XuanTie) <guoren@kernel.org>
---
 Documentation/admin-guide/kernel-parameters.txt |  2 +-
 arch/riscv/include/asm/qspinlock.h              | 24 ++++++++++++++++++++++++
 arch/riscv/kernel/qspinlock_paravirt.c          |  8 ++++++++
 3 files changed, 33 insertions(+), 1 deletion(-)

diff --git a/Documentation/admin-guide/kernel-parameters.txt b/Documentation/admin-guide/kernel-parameters.txt
index 33cd30996e47..d32bb7db9f91 100644
--- a/Documentation/admin-guide/kernel-parameters.txt
+++ b/Documentation/admin-guide/kernel-parameters.txt
@@ -4632,7 +4632,7 @@ Kernel parameters
 			as generic guest with no PV drivers. Currently support
 			XEN HVM, KVM, HYPER_V and VMWARE guest.
 
-	nopvspin	[X86,XEN,KVM,EARLY]
+	nopvspin	[X86,RISCV,XEN,KVM,EARLY]
 			Disables the qspinlock slow path using PV optimizations
 			which allow the hypervisor to 'idle' the guest on lock
 			contention.
diff --git a/arch/riscv/include/asm/qspinlock.h b/arch/riscv/include/asm/qspinlock.h
index 330b714edc44..9c7108baa40a 100644
--- a/arch/riscv/include/asm/qspinlock.h
+++ b/arch/riscv/include/asm/qspinlock.h
@@ -12,6 +12,8 @@
 /* How long a lock should spin before we consider blocking */
 #define SPIN_THRESHOLD		(1 << 15)
 
+extern bool nopvspin;
+
 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);
@@ -29,5 +31,27 @@ static inline void queued_spin_unlock(struct qspinlock *lock)
 #endif /* CONFIG_PARAVIRT_SPINLOCKS */
 
 #include <asm-generic/qspinlock.h>
+#include <asm/jump_label.h>
+
+/*
+ * KVM guests fall back to a Test-and-Set spinlock because fair locks suffer
+ * from severe lock-holder-preemption issues. When virt_spin_lock_key is
+ * enabled, virt_spin_lock() shortcuts queued_spin_lock_slowpath() and hijacks
+ * the lock acquisition.
+ */
+DECLARE_STATIC_KEY_FALSE(virt_spin_lock_key);
+
+#define virt_spin_lock rv_virt_spin_lock
+static inline bool rv_virt_spin_lock(struct qspinlock *lock)
+{
+	if (!static_branch_likely(&virt_spin_lock_key))
+		return false;
+
+	do {
+		smp_cond_load_relaxed((s32 *)&lock->val, VAL == 0);
+	} while (atomic_cmpxchg(&lock->val, 0, _Q_LOCKED_VAL) != 0);
+
+	return true;
+}
 
 #endif /* _ASM_RISCV_QSPINLOCK_H */
diff --git a/arch/riscv/kernel/qspinlock_paravirt.c b/arch/riscv/kernel/qspinlock_paravirt.c
index 04b13994e971..28c6c78d7e84 100644
--- a/arch/riscv/kernel/qspinlock_paravirt.c
+++ b/arch/riscv/kernel/qspinlock_paravirt.c
@@ -50,6 +50,8 @@ 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);
 
+DEFINE_STATIC_KEY_FALSE(virt_spin_lock_key);
+
 bool __init pv_qspinlock_init(void)
 {
 	if (num_possible_cpus() == 1)
@@ -58,6 +60,12 @@ bool __init pv_qspinlock_init(void)
 	if (!sbi_probe_extension(SBI_EXT_PVLOCK))
 		return false;
 
+	if (nopvspin) {
+		static_branch_enable(&virt_spin_lock_key);
+		pr_info("virt_spin_lock enabled by nopvspin\n");
+		return true;
+	}
+
 	pr_info("PV qspinlocks enabled\n");
 	__pv_init_lock_hash();
 

-- 
2.43.0