[PATCH V4] riscv: errata: Add ERRATA_THEAD_WRITE_ONCE fixup

guoren
2026-04-12T14:31:38Z

From: "Guo Ren (Alibaba DAMO Academy)" <guoren@kernel.org>

The early version of XuanTie C910 core has a store merge buffer
delay problem. The store merge buffer could improve the store queue
performance by merging multi-store requests, but when there are not
continued store requests, the prior single store request would be
waiting in the store queue for a long time. That would cause
significant problems for communication between multi-cores. This
problem was found on sg2042 & th1520 platforms with the qspinlock
lock torture test.

So appending a fence w.o could immediately flush the store merge
buffer and let other cores see the write result.

This will apply the WRITE_ONCE errata to handle the non-standard
behavior via appending a fence w.o instruction for WRITE_ONCE().

This problem is only observed on the sg2042 hardware platform by
running the lock_torture test program for half an hour. The problem
was not found in the user space application, because interrupt can
break the livelock.

Reviewed-by: Leonardo Bras <leobras@redhat.com>
Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
Tested-by: Han Gao <gaohan@iscas.ac.cn>
Tested-by: Yao Zi <me@ziyao.cc>
Cc: Chen Wang <unicorn_wang@outlook.com>
Cc: Inochi Amaoto <inochiama@gmail.com>
Cc: Xiaoguang Xing <xiaoguang.xing@sophgo.com>
Cc: Paul Walmsley <pjw@kernel.org>
Signed-off-by: Guo Ren (Alibaba DAMO Academy) <guoren@kernel.org>
---
Changelog

v4:
- Add Reviewed-by: Alexandre Ghiti <alexghiti@rivosinc.com>
(https://lore.kernel.org/linux-riscv/1cfdf6c1-a384-43ad-9588-284335d073f7@ghiti.fr/)
- Add Tested-by: Yao Zi <me@ziyao.cc>
- Rebase on v7.0-rc7

v3:
https://lore.kernel.org/linux-riscv/20260125063941.443777-1-guoren@kernel.org/
- Rebase on 6.19-rc6.
- Remove errata_list_vendors patch which has been merged.
- Add Tested-by tag.

v2:
https://lore.kernel.org/linux-riscv/20250713155321.2064856-1-guoren@kernel.org/
- Add new header file for errata_list_vendors.
- Rebase newest kernel version.

v1:
https://lore.kernel.org/all/20241214143039.4139398-1-guoren@kernel.org/
---
arch/riscv/Kconfig.errata | 17 ++++++++++
arch/riscv/errata/thead/errata.c | 20 ++++++++++++
arch/riscv/include/asm/errata_list_vendors.h | 3 +-
arch/riscv/include/asm/rwonce.h | 34 ++++++++++++++++++++
include/asm-generic/rwonce.h | 2 ++
5 files changed, 75 insertions(+), 1 deletion(-)
create mode 100644 arch/riscv/include/asm/rwonce.h

diff --git a/arch/riscv/Kconfig.errata b/arch/riscv/Kconfig.errata
index 3c945d086c7d..4d3f13522da6 100644
--- a/arch/riscv/Kconfig.errata
+++ b/arch/riscv/Kconfig.errata
@@ -154,4 +154,21 @@ config ERRATA_THEAD_GHOSTWRITE

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

+config ERRATA_THEAD_WRITE_ONCE
+ bool "Apply T-Head WRITE_ONCE errata"
+ depends on ERRATA_THEAD
+ default y
+ help
+ The early version of T-Head C9xx cores of sg2042 & th1520 have a store
+ merge buffer delay problem. The store merge buffer could improve the
+ store queue performance by merging multi-store requests, but when there
+ are no continued store requests, the prior single store request would be
+ waiting in the store queue for a long time. That would cause signifi-
+ cant problems for communication between multi-cores. Appending a
+ fence w.o could immediately flush the store merge buffer and let other
+ cores see the write result.
+
+ This will apply the WRITE_ONCE errata to handle the non-standard beh-
+ avior via appending a fence w.o instruction for WRITE_ONCE().
+
endmenu # "CPU errata selection"
diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
index 0b942183f708..fbe46f2fa8fb 100644
--- a/arch/riscv/errata/thead/errata.c
+++ b/arch/riscv/errata/thead/errata.c
@@ -168,6 +168,23 @@ static bool errata_probe_ghostwrite(unsigned int stage,
return true;
}

+static bool errata_probe_write_once(unsigned int stage,
+ unsigned long arch_id, unsigned long impid)
+{
+ if (!IS_ENABLED(CONFIG_ERRATA_THEAD_WRITE_ONCE))
+ return false;
+
+ /* target-c9xx cores report arch_id and impid as 0 */
+ if (arch_id != 0 || impid != 0)
+ return false;
+
+ if (stage == RISCV_ALTERNATIVES_BOOT ||
+ stage == RISCV_ALTERNATIVES_MODULE)
+ return true;
+
+ return false;
+}
+
static u32 thead_errata_probe(unsigned int stage,
unsigned long archid, unsigned long impid)
{
@@ -183,6 +200,9 @@ static u32 thead_errata_probe(unsigned int stage,

errata_probe_ghostwrite(stage, archid, impid);

+ if (errata_probe_write_once(stage, archid, impid))
+ cpu_req_errata |= BIT(ERRATA_THEAD_WRITE_ONCE);
+
return cpu_req_errata;
}

diff --git a/arch/riscv/include/asm/errata_list_vendors.h b/arch/riscv/include/asm/errata_list_vendors.h
index ec7eba373437..8fd7c36307e2 100644
--- a/arch/riscv/include/asm/errata_list_vendors.h
+++ b/arch/riscv/include/asm/errata_list_vendors.h
@@ -18,7 +18,8 @@
#define ERRATA_THEAD_MAE 0
#define ERRATA_THEAD_PMU 1
#define ERRATA_THEAD_GHOSTWRITE 2
-#define ERRATA_THEAD_NUMBER 3
+#define ERRATA_THEAD_WRITE_ONCE 3
+#define ERRATA_THEAD_NUMBER 4
#endif

#ifdef CONFIG_ERRATA_MIPS
diff --git a/arch/riscv/include/asm/rwonce.h b/arch/riscv/include/asm/rwonce.h
new file mode 100644
index 000000000000..081793d4d772
--- /dev/null
+++ b/arch/riscv/include/asm/rwonce.h
@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+
+#ifndef __ASM_RWONCE_H
+#define __ASM_RWONCE_H
+
+#include <linux/compiler_types.h>
+#include <asm/alternative-macros.h>
+#include <asm/vendorid_list.h>
+#include <asm/errata_list_vendors.h>
+
+#if defined(CONFIG_ERRATA_THEAD_WRITE_ONCE) && !defined(NO_ALTERNATIVE)
+
+#define write_once_fence() \
+do { \
+ asm volatile(ALTERNATIVE( \
+ "nop", \
+ "fence w, o", \
+ THEAD_VENDOR_ID, \
+ ERRATA_THEAD_WRITE_ONCE, \
+ CONFIG_ERRATA_THEAD_WRITE_ONCE) \
+ : : : "memory"); \
+} while (0)
+
+#define __WRITE_ONCE(x, val) \
+do { \
+ *(volatile typeof(x) *)&(x) = (val); \
+ write_once_fence(); \
+} while (0)
+
+#endif /* defined(CONFIG_ERRATA_THEAD_WRITE_ONCE) && !defined(NO_ALTERNATIVE) */
+
+#include <asm-generic/rwonce.h>
+
+#endif /* __ASM_RWONCE_H */
diff --git a/include/asm-generic/rwonce.h b/include/asm-generic/rwonce.h
index 52b969c7cef9..4e2d941f15a1 100644
--- a/include/asm-generic/rwonce.h
+++ b/include/asm-generic/rwonce.h
@@ -50,10 +50,12 @@
__READ_ONCE(x); \
})

+#ifndef __WRITE_ONCE
#define __WRITE_ONCE(x, val) \
do { \
*(volatile typeof(x) *)&(x) = (val); \
} while (0)
+#endif

#define WRITE_ONCE(x, val) \
do { \
--
2.40.1