Re: [PATCH v3 2/2] riscv: Add Zawrs support for spinlocks

[Andrew Jones <ajones@ventanamicro.com>]

On Sun, May 21, 2023 at 01:47:15PM +0200, Heiko Stuebner wrote:
> From: Christoph Müllner <christoph.muellner@vrull.eu>
> 
> The current RISC-V code uses the generic ticket lock implementation,
> that calls the macros smp_cond_load_relaxed() and smp_cond_load_acquire().
> Currently, RISC-V uses the generic implementation of these macros.
> This patch introduces a RISC-V specific implementation, of these
> macros, that peels off the first loop iteration

I guess this peeling off of the first iteration is because it's expected
that the load generated by READ_ONCE() is more efficient than lr.w/d? If
we're worried about unnecessary use of lr.w/d, then shouldn't we look
for a solution that doesn't issue those instructions when we don't have
the Zawrs extension?

> and modifies the waiting
> loop such, that it is possible to use the WRS.STO instruction of the Zawrs
> ISA extension to stall the CPU.
> 
> The resulting implementation of smp_cond_load_*() will only work for
> 32-bit or 64-bit types for RV64 and 32-bit types for RV32.
> This is caused by the restrictions of the LR instruction (RISC-V only
> has LR.W and LR.D). Compiler assertions guard this new restriction.
> 
> This patch uses the existing RISC-V ISA extension framework
> to detect the presence of Zawrs at run-time.
> If available a NOP instruction will be replaced by WRS.NTO or WRS.STO.
> 
> The whole mechanism is gated by Kconfig setting, which defaults to Y.
> 
> The Zawrs specification can be found here:
> https://github.com/riscv/riscv-zawrs/blob/main/zawrs.adoc
> 
> Signed-off-by: Christoph Müllner <christoph.muellner@vrull.eu>
> [rebase, update to review comments]
> Signed-off-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
> ---
>  arch/riscv/Kconfig                   | 10 +++++
>  arch/riscv/include/asm/barrier.h     | 64 ++++++++++++++++++++++++++++
>  arch/riscv/include/asm/errata_list.h | 14 ++++++
>  arch/riscv/include/asm/hwcap.h       |  1 +
>  arch/riscv/kernel/cpu.c              |  1 +
>  arch/riscv/kernel/cpufeature.c       |  1 +
>  6 files changed, 91 insertions(+)
> 
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 348c0fa1fc8c..60ff303ff58a 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -460,6 +460,16 @@ config RISCV_ISA_SVPBMT
>  
>  	   If you don't know what to do here, say Y.
>  
> +config RISCV_ISA_ZAWRS
> +	bool "Zawrs extension support"

This should be "Zawrs extension support for ..."

> +	depends on RISCV_ALTERNATIVE
> +	default y
> +	help
> +	   Adds support to dynamically detect the presence of the Zawrs extension
> +	   (wait for reservation set) and enable its usage.
> +
> +	   If you don't know what to do here, say Y.
> +
>  config TOOLCHAIN_HAS_ZBB
>  	bool
>  	default y
> diff --git a/arch/riscv/include/asm/barrier.h b/arch/riscv/include/asm/barrier.h
> index 110752594228..bd64cfe5ae10 100644
> --- a/arch/riscv/include/asm/barrier.h
> +++ b/arch/riscv/include/asm/barrier.h
> @@ -12,6 +12,8 @@
>  
>  #ifndef __ASSEMBLY__
>  
> +#include <asm/errata_list.h>
> +
>  #define nop()		__asm__ __volatile__ ("nop")
>  #define __nops(n)	".rept	" #n "\nnop\n.endr\n"
>  #define nops(n)		__asm__ __volatile__ (__nops(n))
> @@ -44,6 +46,36 @@ do {									\
>  	___p1;								\
>  })
>  
> +#define ___smp_load_reservedN(pfx, ptr)					\
> +({									\
> +	typeof(*ptr) ___p1;						\
> +	__asm__ __volatile__ ("lr." pfx "	%[p], %[c]\n"		\
> +			      : [p]"=&r" (___p1), [c]"+A"(*ptr));	\
> +	___p1;								\
> +})
> +
> +#define __smp_load_reserved_relaxed(ptr)				\
> +({									\
> +	typeof(*ptr) ___p1;						\
> +	if (sizeof(*ptr) == sizeof(int))				\
> +		___p1 = ___smp_load_reservedN("w", ptr);		\
> +	else if (sizeof(*ptr) == sizeof(long))				\
> +		___p1 = ___smp_load_reservedN("d", ptr);		\
> +	else								\
> +		compiletime_assert(0,					\
> +			"Need type compatible with LR/SC instructions "	\
> +			"for " __stringify(ptr));			\
> +	___p1;								\

It's more common to use a switch for these things, embedding the
lr.w/d asm in each case, something like the macros in
arch/riscv/include/asm/cmpxchg.h. Can we stick with that pattern?

> +})
> +
> +#define __smp_load_reserved_acquire(ptr)				\
> +({									\
> +	typeof(*ptr) ___p1;						\
> +	___p1 = __smp_load_reserved_relaxed(ptr);			\
> +	RISCV_FENCE(r, rw);						\
> +	___p1;								\
> +})
> +
>  /*
>   * This is a very specific barrier: it's currently only used in two places in
>   * the kernel, both in the scheduler.  See include/linux/spinlock.h for the two
> @@ -71,6 +103,38 @@ do {									\
>   */
>  #define smp_mb__after_spinlock()	RISCV_FENCE(iorw,iorw)
>  
> +#define smp_cond_load_relaxed(ptr, cond_expr)				\
> +({									\
> +	typeof(ptr) __PTR = (ptr);					\
> +	__unqual_scalar_typeof(*ptr) VAL;				\
> +	VAL = READ_ONCE(*__PTR);					\
> +	if (!cond_expr) {						\
> +		for (;;) {						\
> +			VAL = __smp_load_reserved_relaxed(__PTR);	\
> +			if (cond_expr)					\
> +				break;					\
> +			ALT_WRS_STO();					\
> +		}							\
> +	}								\
> +	(typeof(*ptr))VAL;						\
> +})
> +
> +#define smp_cond_load_acquire(ptr, cond_expr)				\
> +({									\
> +	typeof(ptr) __PTR = (ptr);					\
> +	__unqual_scalar_typeof(*ptr) VAL;				\
> +	VAL = smp_load_acquire(__PTR);					\
> +	if (!cond_expr) {						\
> +		for (;;) {						\
> +			VAL = __smp_load_reserved_acquire(__PTR);	\
> +			if (cond_expr)					\
> +				break;					\
> +			ALT_WRS_STO();					\
> +		}							\
> +	}								\
> +	(typeof(*ptr))VAL;						\
> +})
> +
>  #include <asm-generic/barrier.h>
>  
>  #endif /* __ASSEMBLY__ */
> diff --git a/arch/riscv/include/asm/errata_list.h b/arch/riscv/include/asm/errata_list.h
> index fb1a810f3d8c..36a72a07d62c 100644
> --- a/arch/riscv/include/asm/errata_list.h
> +++ b/arch/riscv/include/asm/errata_list.h
> @@ -44,6 +44,20 @@ asm(ALTERNATIVE("sfence.vma %0", "sfence.vma", SIFIVE_VENDOR_ID,	\
>  		ERRATA_SIFIVE_CIP_1200, CONFIG_ERRATA_SIFIVE_CIP_1200)	\
>  		: : "r" (addr) : "memory")
>  
> +#define ZAWRS_WRS_NTO	".long 0x00d00073"
> +#define ALT_WRS_NTO()							\
> +asm volatile(ALTERNATIVE(						\
> +	"nop\n\t",							\
> +	ZAWRS_WRS_NTO "\n\t",						\
> +	0, RISCV_ISA_EXT_ZAWRS, CONFIG_RISCV_ISA_ZAWRS))

We don't use this. Do we need to define it now?

> +
> +#define ZAWRS_WRS_STO	".long 0x01d00073"

I'd prefer we use insn-def.h to define instructions, rather than scatter
.long's around, but since this instruction doesn't have any inputs, then
I guess it's not so important to use insn-def.h.

> +#define ALT_WRS_STO()							\
> +asm volatile(ALTERNATIVE(						\
> +	"nop\n\t",							\
> +	ZAWRS_WRS_STO "\n\t",						\
> +	0, RISCV_ISA_EXT_ZAWRS, CONFIG_RISCV_ISA_ZAWRS))
> +

These alternatives aren't supporting errata, so putting them in
errata_list.h doesn't seem appropriate. I think cpufeature.h
would be better.

>  /*
>   * _val is marked as "will be overwritten", so need to set it to 0
>   * in the default case.
> diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> index e0c40a4c63d5..4233d87539ab 100644
> --- a/arch/riscv/include/asm/hwcap.h
> +++ b/arch/riscv/include/asm/hwcap.h
> @@ -46,6 +46,7 @@
>  #define RISCV_ISA_EXT_ZICBOZ		34
>  #define RISCV_ISA_EXT_SMAIA		35
>  #define RISCV_ISA_EXT_SSAIA		36
> +#define RISCV_ISA_EXT_ZAWRS		37
>  
>  #define RISCV_ISA_EXT_MAX		64
>  #define RISCV_ISA_EXT_NAME_LEN_MAX	32
> diff --git a/arch/riscv/kernel/cpu.c b/arch/riscv/kernel/cpu.c
> index c96aa56cf1c7..ac9c604d8936 100644
> --- a/arch/riscv/kernel/cpu.c
> +++ b/arch/riscv/kernel/cpu.c
> @@ -184,6 +184,7 @@ static struct riscv_isa_ext_data isa_ext_arr[] = {
>  	__RISCV_ISA_EXT_DATA(zicbom, RISCV_ISA_EXT_ZICBOM),
>  	__RISCV_ISA_EXT_DATA(zicboz, RISCV_ISA_EXT_ZICBOZ),
>  	__RISCV_ISA_EXT_DATA(zihintpause, RISCV_ISA_EXT_ZIHINTPAUSE),
> +	__RISCV_ISA_EXT_DATA(zawrs, RISCV_ISA_EXT_ZAWRS),
>  	__RISCV_ISA_EXT_DATA(zbb, RISCV_ISA_EXT_ZBB),
>  	__RISCV_ISA_EXT_DATA(smaia, RISCV_ISA_EXT_SMAIA),
>  	__RISCV_ISA_EXT_DATA(ssaia, RISCV_ISA_EXT_SSAIA),
> diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> index b1d6b7e4b829..d4a22a7aed99 100644
> --- a/arch/riscv/kernel/cpufeature.c
> +++ b/arch/riscv/kernel/cpufeature.c
> @@ -236,6 +236,7 @@ void __init riscv_fill_hwcap(void)
>  				SET_ISA_EXT_MAP("svinval", RISCV_ISA_EXT_SVINVAL);
>  				SET_ISA_EXT_MAP("svnapot", RISCV_ISA_EXT_SVNAPOT);
>  				SET_ISA_EXT_MAP("svpbmt", RISCV_ISA_EXT_SVPBMT);
> +				SET_ISA_EXT_MAP("zawrs", RISCV_ISA_EXT_ZAWRS);
>  				SET_ISA_EXT_MAP("zbb", RISCV_ISA_EXT_ZBB);
>  				SET_ISA_EXT_MAP("zicbom", RISCV_ISA_EXT_ZICBOM);
>  				SET_ISA_EXT_MAP("zicboz", RISCV_ISA_EXT_ZICBOZ);
> -- 
> 2.39.0
>

Thanks,
drew

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