On Sun, Sep 10, 2023 at 04:29:03AM -0400, guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
>
> The early version of T-Head C9xx cores 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().
>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> ---
> arch/riscv/Kconfig.errata | 19 +++++++++++++++++++
> arch/riscv/errata/thead/errata.c | 20 ++++++++++++++++++++
> arch/riscv/include/asm/errata_list.h | 13 -------------
> arch/riscv/include/asm/rwonce.h | 24 ++++++++++++++++++++++++
> arch/riscv/include/asm/vendorid_list.h | 14 ++++++++++++++
> include/asm-generic/rwonce.h | 2 ++
> 6 files changed, 79 insertions(+), 13 deletions(-)
> create mode 100644 arch/riscv/include/asm/rwonce.h
>
> diff --git a/arch/riscv/Kconfig.errata b/arch/riscv/Kconfig.errata
> index 1aa85a427ff3..c919cc3f1a3a 100644
> --- a/arch/riscv/Kconfig.errata
> +++ b/arch/riscv/Kconfig.errata
> @@ -77,4 +77,23 @@ config ERRATA_THEAD_PMU
>
> 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 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 no
> + 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. 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().
> +
> + If you don't know what to do here, say "Y".
> +
> endmenu # "CPU errata selection"
> diff --git a/arch/riscv/errata/thead/errata.c b/arch/riscv/errata/thead/errata.c
> index be84b14f0118..751eb5a7f614 100644
> --- a/arch/riscv/errata/thead/errata.c
> +++ b/arch/riscv/errata/thead/errata.c
> @@ -69,6 +69,23 @@ static bool errata_probe_pmu(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)
> {
> @@ -83,6 +100,9 @@ static u32 thead_errata_probe(unsigned int stage,
> if (errata_probe_pmu(stage, archid, impid))
> cpu_req_errata |= BIT(ERRATA_THEAD_PMU);
>
> + 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.h b/arch/riscv/include/asm/errata_list.h
> index 712cab7adffe..fbb2b8d39321 100644
> --- a/arch/riscv/include/asm/errata_list.h
> +++ b/arch/riscv/include/asm/errata_list.h
> @@ -11,19 +11,6 @@
> #include <asm/hwcap.h>
> #include <asm/vendorid_list.h>
>
> -#ifdef CONFIG_ERRATA_SIFIVE
> -#define ERRATA_SIFIVE_CIP_453 0
> -#define ERRATA_SIFIVE_CIP_1200 1
> -#define ERRATA_SIFIVE_NUMBER 2
> -#endif
> -
> -#ifdef CONFIG_ERRATA_THEAD
> -#define ERRATA_THEAD_PBMT 0
> -#define ERRATA_THEAD_CMO 1
> -#define ERRATA_THEAD_PMU 2
> -#define ERRATA_THEAD_NUMBER 3
> -#endif
> -
Here I understand you are moving stuff from errata_list.h to
vendorid_list.h. Wouldn't it be better to do this on a separated patch
before this one?
I understand this is used here, but it looks like it's unrelated.
> #ifdef __ASSEMBLY__
>
> #define ALT_INSN_FAULT(x) \
> diff --git a/arch/riscv/include/asm/rwonce.h b/arch/riscv/include/asm/rwonce.h
> new file mode 100644
> index 000000000000..be0b8864969d
> --- /dev/null
> +++ b/arch/riscv/include/asm/rwonce.h
> @@ -0,0 +1,24 @@
> +/* 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>
> +
> +#define __WRITE_ONCE(x, val) \
> +do { \
> + *(volatile typeof(x) *)&(x) = (val); \
> + asm volatile(ALTERNATIVE( \
> + __nops(1), \
> + "fence w, o\n\t", \
> + THEAD_VENDOR_ID, \
> + ERRATA_THEAD_WRITE_ONCE, \
> + CONFIG_ERRATA_THEAD_WRITE_ONCE) \
> + : : : "memory"); \
> +} while (0)
> +
> +#include <asm-generic/rwonce.h>
> +
> +#endif /* __ASM_RWONCE_H */
IIUC the idea here is to have an alternative __WRITE_ONCE that replaces the
asm-generic one.
Honestly, this asm alternative here seems too much information, and too
cryptic. I mean, yeah in the patch it all makes sense, but I imagine myself
in the future looking at all this and trying to understand what is going
on.
Wouldn't it look better to have something like:
#####
/* Some explanation like the one on Kconfig */
#define write_once_flush() \
do { \
asm volatile(ALTERNATIVE( \
__nops(1), \
"fence w, o\n\t", \
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_flush(); \
} while(0)
#####
This way I could quickly see there is a flush after the writting of
WRITE_ONCE(), and this flush is the above "complicated" asm.
What do you think?
> diff --git a/arch/riscv/include/asm/vendorid_list.h b/arch/riscv/include/asm/vendorid_list.h
> index cb89af3f0704..73078cfe4029 100644
> --- a/arch/riscv/include/asm/vendorid_list.h
> +++ b/arch/riscv/include/asm/vendorid_list.h
> @@ -8,4 +8,18 @@
> #define SIFIVE_VENDOR_ID 0x489
> #define THEAD_VENDOR_ID 0x5b7
>
> +#ifdef CONFIG_ERRATA_SIFIVE
> +#define ERRATA_SIFIVE_CIP_453 0
> +#define ERRATA_SIFIVE_CIP_1200 1
> +#define ERRATA_SIFIVE_NUMBER 2
> +#endif
> +
> +#ifdef CONFIG_ERRATA_THEAD
> +#define ERRATA_THEAD_PBMT 0
> +#define ERRATA_THEAD_CMO 1
> +#define ERRATA_THEAD_PMU 2
> +#define ERRATA_THEAD_WRITE_ONCE 3
> +#define ERRATA_THEAD_NUMBER 4
> +#endif
> +
> #endif
> diff --git a/include/asm-generic/rwonce.h b/include/asm-generic/rwonce.h
> index 8d0a6280e982..fb07fe8c6e45 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 { \
> --
> 2.36.1
>
