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Message-ID: <CAAT7Ki9i5gyvituHsDpeq_-z6=e42MdLEULwqCGJ768-XbVxrQ@mail.gmail.com>
Date: Tue, 15 Jul 2025 21:14:52 +0800
From: Han Gao <rabenda.cn@...il.com>
To: guoren@...nel.org
Cc: palmer@...belt.com, conor@...nel.org, alexghiti@...osinc.com, 
	paul.walmsley@...ive.com, bjorn@...osinc.com, eobras@...hat.com, 
	corbet@....net, peterlin@...estech.com, linux-riscv@...ts.infradead.org, 
	linux-kernel@...r.kernel.org, Leonardo Bras <leobras@...hat.com>
Subject: Re: [PATCH V2 2/2] riscv: errata: Add ERRATA_THEAD_WRITE_ONCE fixup

On Sun, Jul 13, 2025 at 11:53 PM <guoren@...nel.org> wrote:
>
> From: "Guo Ren (Alibaba DAMO Academy)" <guoren@...nel.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@...hat.com>
> Signed-off-by: Guo Ren (Alibaba DAMO Academy) <guoren@...nel.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 e318119d570d..d2c982ba5373 100644
> --- a/arch/riscv/Kconfig.errata
> +++ b/arch/riscv/Kconfig.errata
> @@ -130,4 +130,21 @@ 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 a37d1558f39f..a7473cb8874d 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
>
>  #endif
> 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
>

Tested-by: Han Gao <rabenda.cn@...il.com>

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