[v11,05/10] riscv: lib: vectorize copy_to_user/copy_from_user

Commit Message

Andy Chiu Jan. 15, 2024, 5:59 a.m. UTC

This patch utilizes Vector to perform copy_to_user/copy_from_user. If
Vector is available and the size of copy is large enough for Vector to
perform better than scalar, then direct the kernel to do Vector copies
for userspace. Though the best programming practice for users is to
reduce the copy, this provides a faster variant when copies are
inevitable.

The optimal size for using Vector, copy_to_user_thres, is only a
heuristic for now. We can add DT parsing if people feel the need of
customizing it.

The exception fixup code of the __asm_vector_usercopy must fallback to
the scalar one because accessing user pages might fault, and must be
sleepable. Current kernel-mode Vector does not allow tasks to be
preemptible, so we must disactivate Vector and perform a scalar fallback
in such case.

The original implementation of Vector operations comes from
https://github.com/sifive/sifive-libc, which we agree to contribute to
Linux kernel.

Co-developed-by: Jerry Shih <[e-mail]>
Signed-off-by: Jerry Shih <[e-mail]>
Co-developed-by: Nick Knight <[e-mail]>
Signed-off-by: Nick Knight <[e-mail]>
Suggested-by: Guo Ren <[e-mail]>
Signed-off-by: Andy Chiu <[e-mail]>
---
Changelog v11:
 - pass the proper size when falling back to scalar.
 - Honor the original implementation and authors.
 - Skip bytes which have been processed by the vector store when falling
   back to scalar (Guo)
Changelog v10:
 - remove duplicated code (Charlie)
Changelog v8:
 - fix no-mmu build
Changelog v6:
 - Add a kconfig entry to configure threshold values (Charlie)
 - Refine assembly code (Charlie)
Changelog v4:
 - new patch since v4
---
 arch/riscv/Kconfig                      |  8 ++++
 arch/riscv/include/asm/asm-prototypes.h |  4 ++
 arch/riscv/lib/Makefile                 |  6 ++-
 arch/riscv/lib/riscv_v_helpers.c        | 45 +++++++++++++++++++++
 arch/riscv/lib/uaccess.S                | 10 +++++
 arch/riscv/lib/uaccess_vector.S         | 53 +++++++++++++++++++++++++
 6 files changed, 125 insertions(+), 1 deletion(-)
 create mode 100644 arch/riscv/lib/riscv_v_helpers.c
 create mode 100644 arch/riscv/lib/uaccess_vector.S

Patch

diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
index d42155c29a55..ff48dc2d0dcc 100644
--- a/arch/riscv/Kconfig
+++ b/arch/riscv/Kconfig
@@ -527,6 +527,14 @@  config RISCV_ISA_V_DEFAULT_ENABLE
 
 	  If you don't know what to do here, say Y.
 
+config RISCV_ISA_V_UCOPY_THRESHOLD
+	int "Threshold size for vectorized user copies"
+	depends on RISCV_ISA_V
+	default 768
+	help
+	  Prefer using vectorized copy_to_user()/copy_from_user() when the
+	  workload size exceeds this value.
+
 config TOOLCHAIN_HAS_ZBB
 	bool
 	default y
diff --git a/arch/riscv/include/asm/asm-prototypes.h b/arch/riscv/include/asm/asm-prototypes.h
index 6db1a9bbff4c..be438932f321 100644
--- a/arch/riscv/include/asm/asm-prototypes.h
+++ b/arch/riscv/include/asm/asm-prototypes.h
@@ -11,6 +11,10 @@  long long __ashlti3(long long a, int b);
 
 #ifdef CONFIG_RISCV_ISA_V
 
+#ifdef CONFIG_MMU
+asmlinkage int enter_vector_usercopy(void *dst, void *src, size_t n);
+#endif /* CONFIG_MMU  */
+
 void xor_regs_2_(unsigned long bytes, unsigned long *__restrict p1,
 		 const unsigned long *__restrict p2);
 void xor_regs_3_(unsigned long bytes, unsigned long *__restrict p1,
diff --git a/arch/riscv/lib/Makefile b/arch/riscv/lib/Makefile
index 494f9cd1a00c..c8a6787d5827 100644
--- a/arch/riscv/lib/Makefile
+++ b/arch/riscv/lib/Makefile
@@ -6,9 +6,13 @@  lib-y			+= memmove.o
 lib-y			+= strcmp.o
 lib-y			+= strlen.o
 lib-y			+= strncmp.o
-lib-$(CONFIG_MMU)	+= uaccess.o
+ifeq ($(CONFIG_MMU), y)
+lib-y				+= uaccess.o
+lib-$(CONFIG_RISCV_ISA_V)	+= uaccess_vector.o
+endif
 lib-$(CONFIG_64BIT)	+= tishift.o
 lib-$(CONFIG_RISCV_ISA_ZICBOZ)	+= clear_page.o
 
 obj-$(CONFIG_FUNCTION_ERROR_INJECTION) += error-inject.o
 lib-$(CONFIG_RISCV_ISA_V)	+= xor.o
+lib-$(CONFIG_RISCV_ISA_V)	+= riscv_v_helpers.o
diff --git a/arch/riscv/lib/riscv_v_helpers.c b/arch/riscv/lib/riscv_v_helpers.c
new file mode 100644
index 000000000000..be38a93cedae
--- /dev/null
+++ b/arch/riscv/lib/riscv_v_helpers.c
@@ -0,0 +1,45 @@ 
+// SPDX-License-Identifier: GPL-2.0-or-later
+/*
+ * Copyright (C) 2023 SiFive
+ * Author: Andy Chiu <[e-mail]>
+ */
+#include <linux/linkage.h>
+#include <asm/asm.h>
+
+#include <asm/vector.h>
+#include <asm/simd.h>
+
+#ifdef CONFIG_MMU
+#include <asm/asm-prototypes.h>
+#endif
+
+#ifdef CONFIG_MMU
+size_t riscv_v_usercopy_threshold = CONFIG_RISCV_ISA_V_UCOPY_THRESHOLD;
+int __asm_vector_usercopy(void *dst, void *src, size_t n);
+int fallback_scalar_usercopy(void *dst, void *src, size_t n);
+asmlinkage int enter_vector_usercopy(void *dst, void *src, size_t n)
+{
+	size_t remain, copied;
+
+	/* skip has_vector() check because it has been done by the asm  */
+	if (!may_use_simd())
+		goto fallback;
+
+	kernel_vector_begin();
+	remain = __asm_vector_usercopy(dst, src, n);
+	kernel_vector_end();
+
+	if (remain) {
+		copied = n - remain;
+		dst += copied;
+		src += copied;
+		n = remain;
+		goto fallback;
+	}
+
+	return remain;
+
+fallback:
+	return fallback_scalar_usercopy(dst, src, n);
+}
+#endif
diff --git a/arch/riscv/lib/uaccess.S b/arch/riscv/lib/uaccess.S
index a9d356d6c03c..bc22c078aba8 100644
--- a/arch/riscv/lib/uaccess.S
+++ b/arch/riscv/lib/uaccess.S
@@ -3,6 +3,8 @@ 
 #include <asm/asm.h>
 #include <asm/asm-extable.h>
 #include <asm/csr.h>
+#include <asm/hwcap.h>
+#include <asm/alternative-macros.h>
 
 	.macro fixup op reg addr lbl
 100:
@@ -11,6 +13,13 @@ 
 	.endm
 
 SYM_FUNC_START(__asm_copy_to_user)
+#ifdef CONFIG_RISCV_ISA_V
+	ALTERNATIVE("j fallback_scalar_usercopy", "nop", 0, RISCV_ISA_EXT_v, CONFIG_RISCV_ISA_V)
+	REG_L	t0, riscv_v_usercopy_threshold
+	bltu	a2, t0, fallback_scalar_usercopy
+	tail enter_vector_usercopy
+#endif
+SYM_FUNC_START(fallback_scalar_usercopy)
 
 	/* Enable access to user memory */
 	li t6, SR_SUM
@@ -181,6 +190,7 @@  SYM_FUNC_START(__asm_copy_to_user)
 	sub a0, t5, a0
 	ret
 SYM_FUNC_END(__asm_copy_to_user)
+SYM_FUNC_END(fallback_scalar_usercopy)
 EXPORT_SYMBOL(__asm_copy_to_user)
 SYM_FUNC_ALIAS(__asm_copy_from_user, __asm_copy_to_user)
 EXPORT_SYMBOL(__asm_copy_from_user)
diff --git a/arch/riscv/lib/uaccess_vector.S b/arch/riscv/lib/uaccess_vector.S
new file mode 100644
index 000000000000..51ab5588e9ff
--- /dev/null
+++ b/arch/riscv/lib/uaccess_vector.S
@@ -0,0 +1,53 @@ 
+/* SPDX-License-Identifier: GPL-2.0-only */
+
+#include <linux/linkage.h>
+#include <asm-generic/export.h>
+#include <asm/asm.h>
+#include <asm/asm-extable.h>
+#include <asm/csr.h>
+
+#define pDst a0
+#define pSrc a1
+#define iNum a2
+
+#define iVL a3
+
+#define ELEM_LMUL_SETTING m8
+#define vData v0
+
+	.macro fixup op reg addr lbl
+100:
+	\op \reg, \addr
+	_asm_extable	100b, \lbl
+	.endm
+
+SYM_FUNC_START(__asm_vector_usercopy)
+	/* Enable access to user memory */
+	li	t6, SR_SUM
+	csrs	CSR_STATUS, t6
+
+loop:
+	vsetvli iVL, iNum, e8, ELEM_LMUL_SETTING, ta, ma
+	fixup vle8.v vData, (pSrc), 10f
+	sub iNum, iNum, iVL
+	add pSrc, pSrc, iVL
+	fixup vse8.v vData, (pDst), 11f
+	add pDst, pDst, iVL
+	bnez iNum, loop
+
+	/* Exception fixup for vector load is shared with normal exit */
+10:
+	/* Disable access to user memory */
+	csrc	CSR_STATUS, t6
+	mv	a0, iNum
+	ret
+
+	/* Exception fixup code for vector store. */
+11:
+	/* Undo the subtraction after vle8.v */
+	add	iNum, iNum, iVL
+	/* Make sure the scalar fallback skip already processed bytes */
+	csrr	t2, CSR_VSTART
+	sub	iNum, iNum, t2
+	j	10b
+SYM_FUNC_END(__asm_vector_usercopy)