Re: [PATCH v4 4/5] RISC-V: add infrastructure to allow different str* implementations

["Heiko Stübner" <heiko@sntech.de>]

Hi Andrew,

thanks a lot for taking the time to provide these extensive and really
helpful comments.

Am Dienstag, 10. Januar 2023, 13:13:20 CET schrieb Andrew Jones:
> On Mon, Jan 09, 2023 at 07:17:54PM +0100, Heiko Stuebner wrote:
> > From: Heiko Stuebner <heiko.stuebner@vrull.eu>
> > 
> > Depending on supported extensions on specific RISC-V cores,
> > optimized str* functions might make sense.
> > 
> > This adds basic infrastructure to allow patching the function calls
> > via alternatives later on.
> > 
> > The Linux kernel provides standard implementations for string functions
> > but when architectures want to extend them, they need to provide their
> > own.
> > 
> > The added generic string functions are done in assembler (taken from
> > disassembling the main-kernel functions for now) to allow us to control
> > the used registers and extend them with optimized variants.
> > 
> > Signed-off-by: Heiko Stuebner <heiko.stuebner@vrull.eu>
> > ---
> >  arch/riscv/include/asm/string.h | 10 +++++++++
> >  arch/riscv/kernel/riscv_ksyms.c |  3 +++
> >  arch/riscv/lib/Makefile         |  3 +++
> >  arch/riscv/lib/strcmp.S         | 37 ++++++++++++++++++++++++++++++
> >  arch/riscv/lib/strlen.S         | 28 +++++++++++++++++++++++
> >  arch/riscv/lib/strncmp.S        | 40 +++++++++++++++++++++++++++++++++
> >  arch/riscv/purgatory/Makefile   | 13 +++++++++++
> >  7 files changed, 134 insertions(+)
> >  create mode 100644 arch/riscv/lib/strcmp.S
> >  create mode 100644 arch/riscv/lib/strlen.S
> >  create mode 100644 arch/riscv/lib/strncmp.S
> > 
> > diff --git a/arch/riscv/include/asm/string.h b/arch/riscv/include/asm/string.h
> > index 909049366555..a96b1fea24fe 100644
> > --- a/arch/riscv/include/asm/string.h
> > +++ b/arch/riscv/include/asm/string.h
> > @@ -18,6 +18,16 @@ extern asmlinkage void *__memcpy(void *, const void *, size_t);
> >  #define __HAVE_ARCH_MEMMOVE
> >  extern asmlinkage void *memmove(void *, const void *, size_t);
> >  extern asmlinkage void *__memmove(void *, const void *, size_t);
> > +
> > +#define __HAVE_ARCH_STRCMP
> > +extern asmlinkage int strcmp(const char *cs, const char *ct);
> > +
> > +#define __HAVE_ARCH_STRLEN
> > +extern asmlinkage __kernel_size_t strlen(const char *);
> > +
> > +#define __HAVE_ARCH_STRNCMP
> > +extern asmlinkage int strncmp(const char *cs, const char *ct, size_t count);
> > +
> >  /* For those files which don't want to check by kasan. */
> >  #if defined(CONFIG_KASAN) && !defined(__SANITIZE_ADDRESS__)
> >  #define memcpy(dst, src, len) __memcpy(dst, src, len)
> > diff --git a/arch/riscv/kernel/riscv_ksyms.c b/arch/riscv/kernel/riscv_ksyms.c
> > index 5ab1c7e1a6ed..a72879b4249a 100644
> > --- a/arch/riscv/kernel/riscv_ksyms.c
> > +++ b/arch/riscv/kernel/riscv_ksyms.c
> > @@ -12,6 +12,9 @@
> >  EXPORT_SYMBOL(memset);
> >  EXPORT_SYMBOL(memcpy);
> >  EXPORT_SYMBOL(memmove);
> > +EXPORT_SYMBOL(strcmp);
> > +EXPORT_SYMBOL(strlen);
> > +EXPORT_SYMBOL(strncmp);
> >  EXPORT_SYMBOL(__memset);
> >  EXPORT_SYMBOL(__memcpy);
> >  EXPORT_SYMBOL(__memmove);
> > diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
> > index 25d5c9664e57..6c74b0bedd60 100644
> > --- a/arch/riscv/lib/Makefile
> > +++ b/arch/riscv/lib/Makefile
> > @@ -3,6 +3,9 @@ lib-y			+= delay.o
> >  lib-y			+= memcpy.o
> >  lib-y			+= memset.o
> >  lib-y			+= memmove.o
> > +lib-y			+= strcmp.o
> > +lib-y			+= strlen.o
> > +lib-y			+= strncmp.o
> >  lib-$(CONFIG_MMU)	+= uaccess.o
> >  lib-$(CONFIG_64BIT)	+= tishift.o
> >  
> > diff --git a/arch/riscv/lib/strcmp.S b/arch/riscv/lib/strcmp.S
> > new file mode 100644
> > index 000000000000..94440fb8390c
> > --- /dev/null
> > +++ b/arch/riscv/lib/strcmp.S
> > @@ -0,0 +1,37 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +
> > +#include <linux/linkage.h>
> > +#include <asm/asm.h>
> > +#include <asm-generic/export.h>
> > +
> > +/* int strcmp(const char *cs, const char *ct) */
> > +SYM_FUNC_START(strcmp)
> > +	/*
> > +	 * Returns
> > +	 *   a0 - comparison result, value like strcmp
> > +	 *
> > +	 * Parameters
> > +	 *   a0 - string1
> > +	 *   a1 - string2
> > +	 *
> > +	 * Clobbers
> > +	 *   t0, t1, t2
> > +	 */
> > +	mv	t2, a1
> 
> The above instruction and the 'mv a1, t2' below appear to be attempting
> to preserve a1, but that shouldn't be necessary.

correct and gone now

> > +1:
> > +	lbu	t1, 0(a0)
> > +	lbu	t0, 0(a1)
> 
> I'd rather have t0 be 0(a0) and t1 be 0(a1)

ok

> > +	addi	a0, a0, 1
> > +	addi	a1, a1, 1
> > +	beq	t1, t0, 3f
> > +	li	a0, 1
> > +	bgeu	t1, t0, 2f
> > +	li	a0, -1
> > +2:
> > +	mv	a1, t2
> > +	ret
> > +3:
> > +	bnez	t1, 1b
> > +	li	a0, 0
> > +	j	2b
> 
> For fun I removed one conditional and one unconditional branch (untested)
> 
> 1:
>      lbu     t0, 0(a0)
>      lbu     t1, 0(a1)
>      addi    a0, a0, 1
>      addi    a1, a1, 1
>      bne     t0, t1, 2f
>      bnez    t0, 1b
>      li      a0, 0
>      ret
> 2:
>      slt     a1, t1, t0
>      slli    a1, a1, 1
>      li      a0, -1
>      add     a0, a0, a1
>      ret

yep that
- looks correct
- and also seems to produce correct results

also including your
	  sub     a0, t0, t1
comment from the followup that then produces the same result als the
zbb-variant.

And I've also verified the
- 0, if the s1 and s2 are equal;
- a negative value if s1 is less than s2;
- a positive value if s1 is greater than s2.
return value calling convention with documentation

And added a comment above it, pointing out this fact for the next
person stumbling over this :-)


> > +SYM_FUNC_END(strcmp)
> > diff --git a/arch/riscv/lib/strlen.S b/arch/riscv/lib/strlen.S
> > new file mode 100644
> > index 000000000000..09a7aaff26c8
> > --- /dev/null
> > +++ b/arch/riscv/lib/strlen.S
> > @@ -0,0 +1,28 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +
> > +#include <linux/linkage.h>
> > +#include <asm/asm.h>
> > +#include <asm-generic/export.h>
> > +
> > +/* int strlen(const char *s) */
> > +SYM_FUNC_START(strlen)
> > +	/*
> > +	 * Returns
> > +	 *   a0 - string length
> > +	 *
> > +	 * Parameters
> > +	 *   a0 - String to measure
> > +	 *
> > +	 * Clobbers:
> > +	 *   t0, t1
> > +	 */
> > +	mv	t1, a0
> > +1:
> > +	lbu	t0, 0(t1)
> > +	bnez	t0, 2f
> > +	sub	a0, t1, a0
> > +	ret
> > +2:
> > +	addi	t1, t1, 1
> > +	j	1b
> 
> Slightly reorganizing looks better (to me)
> 
>    mv    t1, a0
> 1:
>    lbu   t0, 0(t1)
>    beqz  t0, 2f
>    addi  t1, t1, 1
>    j     1b
> 2:
>    sub a0, t1, a0
>    ret

ok


> > +SYM_FUNC_END(strlen)
> > diff --git a/arch/riscv/lib/strncmp.S b/arch/riscv/lib/strncmp.S
> > new file mode 100644
> > index 000000000000..493ab6febcb2
> > --- /dev/null
> > +++ b/arch/riscv/lib/strncmp.S
> > @@ -0,0 +1,40 @@
> > +/* SPDX-License-Identifier: GPL-2.0-only */
> > +
> > +#include <linux/linkage.h>
> > +#include <asm/asm.h>
> > +#include <asm-generic/export.h>
> > +
> > +/* int strncmp(const char *cs, const char *ct, size_t count) */
> > +SYM_FUNC_START(strncmp)
> > +	/*
> > +	 * Returns
> > +	 *   a0 - comparison result, value like strncmp
> > +	 *
> > +	 * Parameters
> > +	 *   a0 - string1
> > +	 *   a1 - string2
> > +	 *   a2 - number of characters to compare
> > +	 *
> > +	 * Clobbers
> > +	 *   t0, t1, t2
> > +	 */
> > +	li	t0, 0
> > +1:
> > +	beq	a2, t0, 4f
> > +	add	t1, a0, t0
> > +	add	t2, a1, t0
> > +	lbu	t1, 0(t1)
> > +	lbu	t2, 0(t2)
> > +	beq	t1, t2, 3f
> > +	li	a0, 1
> > +	bgeu	t1, t2, 2f
> > +	li	a0, -1
> > +2:
> > +	ret
> > +3:
> > +	addi	t0, t0, 1
> > +	bnez	t1, 1b
> > +4:
> > +	li	a0, 0
> > +	j	2b
> 
> (untested)
> 
>      li      t2, 0
> 1:
>      beq     a2, t2, 2f
>      lbu     t0, 0(a0)
>      lbu     t1, 0(a1)
>      addi    a0, a0, 1
>      addi    a1, a1, 1
>      bne     t0, t1, 3f
>      addi    t2, t2, 1
>      bnez    t0, 1b
> 2:
>      li      a0, 0
>      ret
> 3:
>      slt     a1, t1, t0
>      slli    a1, a1, 1
>      li      a0, -1
>      add     a0, a0, a1
>      ret

same here, I did go over the changed assembly and verified that
it produces the same results as the original and then did a second
pass to also add the  
	sub     a0, t0, t1
replacment.


> > +SYM_FUNC_END(strncmp)
> > diff --git a/arch/riscv/purgatory/Makefile b/arch/riscv/purgatory/Makefile
> > index dd58e1d99397..d16bf715a586 100644
> > --- a/arch/riscv/purgatory/Makefile
> > +++ b/arch/riscv/purgatory/Makefile
> > @@ -2,6 +2,7 @@
> >  OBJECT_FILES_NON_STANDARD := y
> >  
> >  purgatory-y := purgatory.o sha256.o entry.o string.o ctype.o memcpy.o memset.o
> > +purgatory-y += strcmp.o strlen.o strncmp.o
> >  
> >  targets += $(purgatory-y)
> >  PURGATORY_OBJS = $(addprefix $(obj)/,$(purgatory-y))
> > @@ -18,6 +19,15 @@ $(obj)/memcpy.o: $(srctree)/arch/riscv/lib/memcpy.S FORCE
> >  $(obj)/memset.o: $(srctree)/arch/riscv/lib/memset.S FORCE
> >  	$(call if_changed_rule,as_o_S)
> >  
> > +$(obj)/strcmp.o: $(srctree)/arch/riscv/lib/strcmp.S FORCE
> > +	$(call if_changed_rule,as_o_S)
> > +
> > +$(obj)/strlen.o: $(srctree)/arch/riscv/lib/strlen.S FORCE
> > +	$(call if_changed_rule,as_o_S)
> > +
> > +$(obj)/strncmp.o: $(srctree)/arch/riscv/lib/strncmp.S FORCE
> > +	$(call if_changed_rule,as_o_S)
> > +
> >  $(obj)/sha256.o: $(srctree)/lib/crypto/sha256.c FORCE
> >  	$(call if_changed_rule,cc_o_c)
> >  
> > @@ -77,6 +87,9 @@ CFLAGS_ctype.o			+= $(PURGATORY_CFLAGS)
> >  AFLAGS_REMOVE_entry.o		+= -Wa,-gdwarf-2
> >  AFLAGS_REMOVE_memcpy.o		+= -Wa,-gdwarf-2
> >  AFLAGS_REMOVE_memset.o		+= -Wa,-gdwarf-2
> > +AFLAGS_REMOVE_strcmp.o		+= -Wa,-gdwarf-2
> > +AFLAGS_REMOVE_strlen.o		+= -Wa,-gdwarf-2
> > +AFLAGS_REMOVE_strncmp.o		+= -Wa,-gdwarf-2
> >  
> >  $(obj)/purgatory.ro: $(PURGATORY_OBJS) FORCE
> >  		$(call if_changed,ld)
> >
> 
> With at least the removal of the unnecessary preserving of a1 in strcmp,
> then it looks correct to me, so
> 
> Reviewed-by: Andrew Jones <ajones@ventanamicro.com>
> 
> But I think there's room for making it more readable, and maybe even
> optimized, as I've tried to do.


Thanks a lot
Heiko



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