Re: [PATCH V11 09/17] riscv: qspinlock: errata: Add ERRATA_THEAD_WRITE_ONCE fixup

[Leonardo Bras <leobras@redhat.com>]

On Sun, Sep 10, 2023 at 04:29:03AM -0400, guoren@kernel.org wrote:
> From: Guo Ren <guoren@linux.alibaba.com>
> 
> 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@linux.alibaba.com>
> Signed-off-by: Guo Ren <guoren@kernel.org>
> ---
>  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
>