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Mon, 13 Apr 2026 00:41:02 -0700 (PDT) Date: Mon, 13 Apr 2026 15:40:58 +0800 From: Inochi Amaoto To: guoren@kernel.org, paul.walmsley@sifive.com, palmer@dabbelt.com, alex@ghiti.fr, leobras@redhat.com, djordje.todorovic@htecgroup.com, aleksa.paunovic@htecgroup.com, arnd@arndb.de, rabenda.cn@gmail.com Cc: linux-riscv@lists.infradead.org, linux-kernel@vger.kernel.org, linux-arch@vger.kernel.org, Alexandre Ghiti , Han Gao , Yao Zi , Chen Wang , Inochi Amaoto , Xiaoguang Xing , Paul Walmsley Subject: Re: [PATCH V4] riscv: errata: Add ERRATA_THEAD_WRITE_ONCE fixup Message-ID: References: <20260412143116.1445893-1-guoren@kernel.org> Precedence: bulk X-Mailing-List: linux-arch@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20260412143116.1445893-1-guoren@kernel.org> On Sun, Apr 12, 2026 at 10:31:16AM -0400, guoren@kernel.org wrote: > From: "Guo Ren (Alibaba DAMO Academy)" > > 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 > Reviewed-by: Alexandre Ghiti > Tested-by: Han Gao > Tested-by: Yao Zi > Cc: Chen Wang > Cc: Inochi Amaoto > Cc: Xiaoguang Xing > Cc: Paul Walmsley > Signed-off-by: Guo Ren (Alibaba DAMO Academy) > --- Reviewed-by: Inochi Amaoto > Changelog > > v4: > - Add Reviewed-by: Alexandre Ghiti > (https://lore.kernel.org/linux-riscv/1cfdf6c1-a384-43ad-9588-284335d073f7@ghiti.fr/) > - Add Tested-by: Yao Zi > - 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 > +#include > +#include > +#include > + > +#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 > + > +#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 >