[v8, 05/10] riscv: lib: vectorize copy_to_user/copy_from_user
Andy Chiu
andy.chiu at sifive.com
Tue Dec 26 19:15:19 PST 2023
On Wed, Dec 27, 2023 at 9:34 AM Guo Ren <guoren at kernel.org> wrote:
>
> On Sat, Dec 23, 2023 at 12:30 PM Andy Chiu <andy.chiu at sifive.com> wrote:
> >
> > 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.
> >
> > Signed-off-by: Andy Chiu <andy.chiu at sifive.com>
> > ---
> > 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 | 44 ++++++++++++++++++++++
> > arch/riscv/lib/uaccess.S | 10 +++++
> > arch/riscv/lib/uaccess_vector.S | 50 +++++++++++++++++++++++++
> > 6 files changed, 121 insertions(+), 1 deletion(-)
> > create mode 100644 arch/riscv/lib/riscv_v_helpers.c
> > create mode 100644 arch/riscv/lib/uaccess_vector.S
> >
> > diff --git a/arch/riscv/Kconfig b/arch/riscv/Kconfig
> > index 95a2a06acc6a..3c5ba05e8a2d 100644
> > --- a/arch/riscv/Kconfig
> > +++ b/arch/riscv/Kconfig
> > @@ -525,6 +525,14 @@ 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..6cac8f4e69e9
> > --- /dev/null
> > +++ b/arch/riscv/lib/riscv_v_helpers.c
> > @@ -0,0 +1,44 @@
> > +// SPDX-License-Identifier: GPL-2.0-or-later
> > +/*
> > + * Copyright (C) 2023 SiFive
> > + * Author: Andy Chiu <andy.chiu at sifive.com>
> > + */
> > +#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;
> > + 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 3ab438f30d13..a1e4a3c42925 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..7bd96cee39e4
> > --- /dev/null
> > +++ b/arch/riscv/lib/uaccess_vector.S
> > @@ -0,0 +1,50 @@
> > +/* 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
> > + fixup vse8.v vData, (pDst), 10f
> > + sub iNum, iNum, iVL
> > + add pSrc, pSrc, iVL
> > + add pDst, pDst, iVL
> > + bnez iNum, loop
> > +
> > +.Lout_copy_user:
> > + /* Disable access to user memory */
> > + csrc CSR_STATUS, t6
> > + li a0, 0
> > + ret
> > +
> > + /* Exception fixup code */
> > +10:
> > + /* Disable access to user memory */
> > + csrc CSR_STATUS, t6
> > + mv a0, iNum
> Shall we check CSR_VSTART to find out how many elements were copied?
This is a good idea! But is it possible to find out if we were trapped
at the load or the store instruction? IIUC if we trap at the load then
we could not derive the number of copied bytes from CSR_VSTART.
Thanks,
Andy
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