Re: [PATCH 2/2] riscv: select DCACHE_WORD_ACCESS for efficient unaligned access HW

Conor Dooley
2023-12-03T11:53:40Z

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On Sat, Dec 02, 2023 at 07:18:22PM +0800, Jisheng Zhang wrote:
> DCACHE_WORD_ACCESS uses the word-at-a-time API for optimised string
> comparisons in the vfs layer.
>
> This patch implements support for load_unaligned_zeropad in much the
> same way as has been done for arm64.
>
> Here is the test program and step:
>
> $ cat tt.c
> #include <sys/types.h>
> #include <sys/stat.h>
> #include <unistd.h>
>
> #define ITERATIONS 1000000
>
> #define PATH "123456781234567812345678123456781"
>
> int main(void)
> {
> unsigned long i;
> struct stat buf;
>
> for (i = 0; i < ITERATIONS; i++)
> stat(PATH, &buf);
>
> return 0;
> }
>
> $ gcc -O2 tt.c
> $ touch 123456781234567812345678123456781
> $ time ./a.out
>
> Per my test on T-HEAD C910 platforms, the above test performance is
> improved by about 7.5%.
>
> Signed-off-by: Jisheng Zhang <jszhang@kernel.org>

Doesn't build on nommu:
arch/riscv/include/asm/word-at-a-time.h:64:9: error: expected ':' or ')' before '_ASM_EXTABLE_LOAD_UNALIGNED_ZEROPAD'
arch/riscv/include/asm/word-at-a-time.h:64:45: error: invalid suffix "b" on integer constant
arch/riscv/include/asm/word-at-a-time.h:64:49: error: invalid suffix "b" on integer constant
arch/riscv/include/asm/word-at-a-time.h:64:9: error: expected ':' or ')' before '_ASM_EXTABLE_LOAD_UNALIGNED_ZEROPAD'
arch/riscv/include/asm/word-at-a-time.h:64:45: error: invalid suffix "b" on integer constant
arch/riscv/include/asm/word-at-a-time.h:64:49: error: invalid suffix "b" on integer constant
arch/riscv/include/asm/word-at-a-time.h:64:9: error: expected ':' or ')' before '_ASM_EXTABLE_LOAD_UNALIGNED_ZEROPAD'
arch/riscv/include/asm/word-at-a-time.h:64:45: error: invalid suffix "b" on integer constant
arch/riscv/include/asm/word-at-a-time.h:64:49: error: invalid suffix "b" on integer constant
arch/riscv/include/asm/word-at-a-time.h:64:9: error: expected ':' or ')' before '_ASM_EXTABLE_LOAD_UNALIGNED_ZEROPAD'
arch/riscv/include/asm/word-at-a-time.h:64:45: error: invalid suffix "b" on integer constant
arch/riscv/include/asm/word-at-a-time.h:64:49: error: invalid suffix "b" on integer constant

Cheers,
Conor.

> ---
> arch/riscv/Kconfig | 1 +
> arch/riscv/include/asm/asm-extable.h | 15 ++++++++++++
> arch/riscv/include/asm/word-at-a-time.h | 23 ++++++++++++++++++
> arch/riscv/mm/extable.c | 31 +++++++++++++++++++++++++
> 4 files changed, 70 insertions(+)
>
> diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> index 0a76209e9b02..bb366eb1870e 100644
> --- a/arch/riscv/Kconfig
> +++ b/arch/riscv/Kconfig
> @@ -657,6 +657,7 @@ config RISCV_MISALIGNED
> config RISCV_EFFICIENT_UNALIGNED_ACCESS
> bool "Use unaligned access for some functions"
> depends on NONPORTABLE
> + select DCACHE_WORD_ACCESS if MMU
> select HAVE_EFFICIENT_UNALIGNED_ACCESS
> default n
> help
> diff --git a/arch/riscv/include/asm/asm-extable.h b/arch/riscv/include/asm/asm-extable.h
> index 00a96e7a9664..0c8bfd54fc4e 100644
> --- a/arch/riscv/include/asm/asm-extable.h
> +++ b/arch/riscv/include/asm/asm-extable.h
> @@ -6,6 +6,7 @@
> #define EX_TYPE_FIXUP 1
> #define EX_TYPE_BPF 2
> #define EX_TYPE_UACCESS_ERR_ZERO 3
> +#define EX_TYPE_LOAD_UNALIGNED_ZEROPAD 4
>
> #ifdef CONFIG_MMU
>
> @@ -47,6 +48,11 @@
> #define EX_DATA_REG_ZERO_SHIFT 5
> #define EX_DATA_REG_ZERO GENMASK(9, 5)
>
> +#define EX_DATA_REG_DATA_SHIFT 0
> +#define EX_DATA_REG_DATA GENMASK(4, 0)
> +#define EX_DATA_REG_ADDR_SHIFT 5
> +#define EX_DATA_REG_ADDR GENMASK(9, 5)
> +
> #define EX_DATA_REG(reg, gpr) \
> "((.L__gpr_num_" #gpr ") << " __stringify(EX_DATA_REG_##reg##_SHIFT) ")"
>
> @@ -62,6 +68,15 @@
> #define _ASM_EXTABLE_UACCESS_ERR(insn, fixup, err) \
> _ASM_EXTABLE_UACCESS_ERR_ZERO(insn, fixup, err, zero)
>
> +#define _ASM_EXTABLE_LOAD_UNALIGNED_ZEROPAD(insn, fixup, data, addr) \
> + __DEFINE_ASM_GPR_NUMS \
> + __ASM_EXTABLE_RAW(#insn, #fixup, \
> + __stringify(EX_TYPE_LOAD_UNALIGNED_ZEROPAD), \
> + "(" \
> + EX_DATA_REG(DATA, data) " | " \
> + EX_DATA_REG(ADDR, addr) \
> + ")")
> +
> #endif /* __ASSEMBLY__ */
>
> #else /* CONFIG_MMU */
> diff --git a/arch/riscv/include/asm/word-at-a-time.h b/arch/riscv/include/asm/word-at-a-time.h
> index 7c086ac6ecd4..5a3865ac3623 100644
> --- a/arch/riscv/include/asm/word-at-a-time.h
> +++ b/arch/riscv/include/asm/word-at-a-time.h
> @@ -9,6 +9,7 @@
> #define _ASM_RISCV_WORD_AT_A_TIME_H
>
>
> +#include <asm/asm-extable.h>
> #include <linux/kernel.h>
>
> struct word_at_a_time {
> @@ -45,4 +46,26 @@ static inline unsigned long find_zero(unsigned long mask)
> /* The mask we created is directly usable as a bytemask */
> #define zero_bytemask(mask) (mask)
>
> +/*
> + * Load an unaligned word from kernel space.
> + *
> + * In the (very unlikely) case of the word being a page-crosser
> + * and the next page not being mapped, take the exception and
> + * return zeroes in the non-existing part.
> + */
> +static inline unsigned long load_unaligned_zeropad(const void *addr)
> +{
> + unsigned long ret;
> +
> + /* Load word from unaligned pointer addr */
> + asm(
> + "1: " REG_L " %0, %2\n"
> + "2:\n"
> + _ASM_EXTABLE_LOAD_UNALIGNED_ZEROPAD(1b, 2b, %0, %1)
> + : "=&r" (ret)
> + : "r" (addr), "m" (*(unsigned long *)addr));
> +
> + return ret;
> +}
> +
> #endif /* _ASM_RISCV_WORD_AT_A_TIME_H */
> diff --git a/arch/riscv/mm/extable.c b/arch/riscv/mm/extable.c
> index 35484d830fd6..dd1530af3ef1 100644
> --- a/arch/riscv/mm/extable.c
> +++ b/arch/riscv/mm/extable.c
> @@ -27,6 +27,14 @@ static bool ex_handler_fixup(const struct exception_table_entry *ex,
> return true;
> }
>
> +static inline unsigned long regs_get_gpr(struct pt_regs *regs, unsigned int offset)
> +{
> + if (unlikely(!offset || offset > MAX_REG_OFFSET))
> + return 0;
> +
> + return *(unsigned long *)((unsigned long)regs + offset);
> +}
> +
> static inline void regs_set_gpr(struct pt_regs *regs, unsigned int offset,
> unsigned long val)
> {
> @@ -50,6 +58,27 @@ static bool ex_handler_uaccess_err_zero(const struct exception_table_entry *ex,
> return true;
> }
>
> +static bool
> +ex_handler_load_unaligned_zeropad(const struct exception_table_entry *ex,
> + struct pt_regs *regs)
> +{
> + int reg_data = FIELD_GET(EX_DATA_REG_DATA, ex->data);
> + int reg_addr = FIELD_GET(EX_DATA_REG_ADDR, ex->data);
> + unsigned long data, addr, offset;
> +
> + addr = regs_get_gpr(regs, reg_addr * sizeof(unsigned long));
> +
> + offset = addr & 0x7UL;
> + addr &= ~0x7UL;
> +
> + data = *(unsigned long *)addr >> (offset * 8);
> +
> + regs_set_gpr(regs, reg_data * sizeof(unsigned long), data);
> +
> + regs->epc = get_ex_fixup(ex);
> + return true;
> +}
> +
> bool fixup_exception(struct pt_regs *regs)
> {
> const struct exception_table_entry *ex;
> @@ -65,6 +94,8 @@ bool fixup_exception(struct pt_regs *regs)
> return ex_handler_bpf(ex, regs);
> case EX_TYPE_UACCESS_ERR_ZERO:
> return ex_handler_uaccess_err_zero(ex, regs);
> + case EX_TYPE_LOAD_UNALIGNED_ZEROPAD:
> + return ex_handler_load_unaligned_zeropad(ex, regs);
> }
>
> BUG();
> --
> 2.42.0
>

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