[PATCH v2 2/2] riscv: select DCACHE_WORD_ACCESS for efficient unaligned access HW

Jisheng Zhang
2023-12-03T14:10:39Z

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>
---
arch/riscv/Kconfig | 1 +
arch/riscv/include/asm/asm-extable.h | 15 ++++++++++++
arch/riscv/include/asm/word-at-a-time.h | 27 +++++++++++++++++++++
arch/riscv/mm/extable.c | 31 +++++++++++++++++++++++++
4 files changed, 74 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..f3f031e34191 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 +46,30 @@ static inline unsigned long find_zero(unsigned long mask)
/* The mask we created is directly usable as a bytemask */
#define zero_bytemask(mask) (mask)

+#ifdef CONFIG_DCACHE_WORD_ACCESS
+
+/*
+ * 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 /* CONFIG_DCACHE_WORD_ACCESS */
+
#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