Re: [PATCH V11 03/17] riscv: Use Zicbop in arch_xchg when available

[Leonardo Bras <leobras@redhat.com>]

On Thu, Sep 14, 2023 at 03:47:59PM +0200, Andrew Jones wrote:
> On Sun, Sep 10, 2023 at 04:28:57AM -0400, guoren@kernel.org wrote:
> > From: Guo Ren <guoren@linux.alibaba.com>
> > 
> > Cache-block prefetch instructions are HINTs to the hardware to
> > indicate that software intends to perform a particular type of
> > memory access in the near future. Enable ARCH_HAS_PREFETCHW and
> > improve the arch_xchg for qspinlock xchg_tail.
> > 
> > Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
> > Signed-off-by: Guo Ren <guoren@kernel.org>
> > ---
> >  arch/riscv/Kconfig                 | 15 +++++++++++++++
> >  arch/riscv/include/asm/cmpxchg.h   |  4 +++-
> >  arch/riscv/include/asm/hwcap.h     |  1 +
> >  arch/riscv/include/asm/insn-def.h  |  5 +++++
> >  arch/riscv/include/asm/processor.h | 13 +++++++++++++
> >  arch/riscv/kernel/cpufeature.c     |  1 +
> >  6 files changed, 38 insertions(+), 1 deletion(-)
> > 
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index e9ae6fa232c3..2c346fe169c1 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -617,6 +617,21 @@ config RISCV_ISA_ZICBOZ
> >  
> >  	   If you don't know what to do here, say Y.
> >  
> > +config RISCV_ISA_ZICBOP
> > +	bool "Zicbop extension support for cache block prefetch"
> > +	depends on MMU
> > +	depends on RISCV_ALTERNATIVE
> > +	default y
> > +	help
> > +	   Adds support to dynamically detect the presence of the ZICBOP
> > +	   extension (Cache Block Prefetch Operations) and enable its
> > +	   usage.
> > +
> > +	   The Zicbop extension can be used to prefetch cache block for
> > +	   read/write/instruction fetch.
> > +
> > +	   If you don't know what to do here, say Y.
> > +
> >  config TOOLCHAIN_HAS_ZIHINTPAUSE
> >  	bool
> >  	default y
> > diff --git a/arch/riscv/include/asm/cmpxchg.h b/arch/riscv/include/asm/cmpxchg.h
> > index 702725727671..56eff7a9d2d2 100644
> > --- a/arch/riscv/include/asm/cmpxchg.h
> > +++ b/arch/riscv/include/asm/cmpxchg.h
> > @@ -11,6 +11,7 @@
> >  
> >  #include <asm/barrier.h>
> >  #include <asm/fence.h>
> > +#include <asm/processor.h>
> >  
> >  #define __arch_xchg_masked(prepend, append, r, p, n)			\
> >  ({									\
> > @@ -25,6 +26,7 @@
> >  									\
> >  	__asm__ __volatile__ (						\
> >  	       prepend							\
> > +	       PREFETCHW_ASM(%5)					\
> >  	       "0:	lr.w %0, %2\n"					\
> >  	       "	and  %1, %0, %z4\n"				\
> >  	       "	or   %1, %1, %z3\n"				\
> > @@ -32,7 +34,7 @@
> >  	       "	bnez %1, 0b\n"					\
> >  	       append							\
> >  	       : "=&r" (__retx), "=&r" (__rc), "+A" (*(__ptr32b))	\
> > -	       : "rJ" (__newx), "rJ" (~__mask)				\
> > +	       : "rJ" (__newx), "rJ" (~__mask), "rJ" (__ptr32b)		\
> >  	       : "memory");						\
> >  									\
> >  	r = (__typeof__(*(p)))((__retx & __mask) >> __s);		\
> > diff --git a/arch/riscv/include/asm/hwcap.h b/arch/riscv/include/asm/hwcap.h
> > index b7b58258f6c7..78b7b8b53778 100644
> > --- a/arch/riscv/include/asm/hwcap.h
> > +++ b/arch/riscv/include/asm/hwcap.h
> > @@ -58,6 +58,7 @@
> >  #define RISCV_ISA_EXT_ZICSR		40
> >  #define RISCV_ISA_EXT_ZIFENCEI		41
> >  #define RISCV_ISA_EXT_ZIHPM		42
> > +#define RISCV_ISA_EXT_ZICBOP		43
> >  
> >  #define RISCV_ISA_EXT_MAX		64
> >  
> > diff --git a/arch/riscv/include/asm/insn-def.h b/arch/riscv/include/asm/insn-def.h
> > index 6960beb75f32..dc590d331894 100644
> > --- a/arch/riscv/include/asm/insn-def.h
> > +++ b/arch/riscv/include/asm/insn-def.h
> > @@ -134,6 +134,7 @@
> >  
> >  #define RV_OPCODE_MISC_MEM	RV_OPCODE(15)
> >  #define RV_OPCODE_SYSTEM	RV_OPCODE(115)
> > +#define RV_OPCODE_PREFETCH	RV_OPCODE(19)
> 
> This should be named RV_OPCODE_OP_IMM and be placed in
> numerical order with the others, i.e. above SYSTEM.
> 
> >  
> >  #define HFENCE_VVMA(vaddr, asid)				\
> >  	INSN_R(OPCODE_SYSTEM, FUNC3(0), FUNC7(17),		\
> > @@ -196,4 +197,8 @@
> >  	INSN_I(OPCODE_MISC_MEM, FUNC3(2), __RD(0),		\
> >  	       RS1(base), SIMM12(4))
> >  
> > +#define CBO_prefetchw(base)					\
> 
> Please name this 'PREFETCH_w' and it should take an immediate parameter,
> even if we intend to pass 0 for it.

It makes sense.

The mnemonic in the previously mentioned documentation is:

prefetch.w offset(base)

So yeah, makes sense to have both offset and base as parameters for 
CBO_prefetchw (or PREFETCH_w, I have no strong preference).

> 
> > +	INSN_R(OPCODE_PREFETCH, FUNC3(6), FUNC7(0),		\
> > +	       RD(x0), RS1(base), RS2(x0))
> 
> prefetch.w is not an R-type instruction, it's an S-type. While the bit
> shifts are the same, the names are different. We need to add S-type
> names while defining this instruction. 

That is correct, it is supposed to look like a store instruction (S-type), 
even though documentation don't explicitly state that.

Even though it works fine with the R-type definition, code documentation 
would be wrong, and future changes could break it.

> Then, this define would be
> 
>  #define PREFETCH_w(base, imm) \
>      INSN_S(OPCODE_OP_IMM, FUNC3(6), IMM_11_5(imm), __IMM_4_0(0), \
>             RS1(base), __RS2(3))

s/OPCODE_OP_IMM/OPCODE_PREFETCH
0x4 vs 0x13

RS2 == 0x3 is correct (PREFETCH.W instead of PREFETCH.I)


So IIUC, it should be:

INSN_S(OPCODE_PREFETCH, FUNC3(6), IMM_11_5(imm), __IMM_4_0(0), \
       RS1(base), __RS2(3)

Thanks,
Leo


> 
> When the assembler as insn_r I hope it will validate that
> (imm & 0xfe0) == imm
> 
> > +
> >  #endif /* __ASM_INSN_DEF_H */
> > diff --git a/arch/riscv/include/asm/processor.h b/arch/riscv/include/asm/processor.h
> > index de9da852f78d..7ad3a24212e8 100644
> > --- a/arch/riscv/include/asm/processor.h
> > +++ b/arch/riscv/include/asm/processor.h
> > @@ -12,6 +12,8 @@
> >  #include <vdso/processor.h>
> >  
> >  #include <asm/ptrace.h>
> > +#include <asm/insn-def.h>
> > +#include <asm/hwcap.h>
> >  
> >  #ifdef CONFIG_64BIT
> >  #define DEFAULT_MAP_WINDOW	(UL(1) << (MMAP_VA_BITS - 1))
> > @@ -103,6 +105,17 @@ static inline void arch_thread_struct_whitelist(unsigned long *offset,
> >  #define KSTK_EIP(tsk)		(ulong)(task_pt_regs(tsk)->epc)
> >  #define KSTK_ESP(tsk)		(ulong)(task_pt_regs(tsk)->sp)
> >  
> > +#define ARCH_HAS_PREFETCHW
> > +#define PREFETCHW_ASM(base)	ALTERNATIVE(__nops(1), \
> > +					    CBO_prefetchw(base), \
> > +					    0, \
> > +					    RISCV_ISA_EXT_ZICBOP, \
> > +					    CONFIG_RISCV_ISA_ZICBOP)
> > +static inline void prefetchw(const void *ptr)
> > +{
> > +	asm volatile(PREFETCHW_ASM(%0)
> > +		: : "r" (ptr) : "memory");
> > +}
> >  
> >  /* Do necessary setup to start up a newly executed thread. */
> >  extern void start_thread(struct pt_regs *regs,
> > diff --git a/arch/riscv/kernel/cpufeature.c b/arch/riscv/kernel/cpufeature.c
> > index ef7b4fd9e876..e0b897db0b97 100644
> > --- a/arch/riscv/kernel/cpufeature.c
> > +++ b/arch/riscv/kernel/cpufeature.c
> > @@ -159,6 +159,7 @@ const struct riscv_isa_ext_data riscv_isa_ext[] = {
> >  	__RISCV_ISA_EXT_DATA(h, RISCV_ISA_EXT_h),
> >  	__RISCV_ISA_EXT_DATA(zicbom, RISCV_ISA_EXT_ZICBOM),
> >  	__RISCV_ISA_EXT_DATA(zicboz, RISCV_ISA_EXT_ZICBOZ),
> > +	__RISCV_ISA_EXT_DATA(zicbop, RISCV_ISA_EXT_ZICBOP),
> 
> zicbop should be above zicboz (extensions alphabetical within their
> category).
> 
> >  	__RISCV_ISA_EXT_DATA(zicntr, RISCV_ISA_EXT_ZICNTR),
> >  	__RISCV_ISA_EXT_DATA(zicsr, RISCV_ISA_EXT_ZICSR),
> >  	__RISCV_ISA_EXT_DATA(zifencei, RISCV_ISA_EXT_ZIFENCEI),
> > -- 
> > 2.36.1
> >
> 
> Thanks,
> drew
>