Re: [PATCHv8 02/17] x86/asm: Introduce inline memcpy and memset

David Laight
2025-07-03T08:44:35Z

On Tue, 1 Jul 2025 12:58:31 +0300
"Kirill A. Shutemov" <kirill.shutemov@linux.intel.com> wrote:

> Extract memcpy and memset functions from copy_user_generic() and
> __clear_user().
>
> They can be used as inline memcpy and memset instead of the GCC builtins
> whenever necessary. LASS requires them to handle text_poke.

Except they contain the fault handlers so aren't generic calls.

>
> Originally-by: Peter Zijlstra <peterz@infradead.org>
> Link: https://lore.kernel.org/all/20241029184840.GJ14555@noisy.programming.kicks-ass.net/
> Signed-off-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> ---
> arch/x86/include/asm/string.h | 46 +++++++++++++++++++++++++++++++
> arch/x86/include/asm/uaccess_64.h | 38 +++++++------------------
> arch/x86/lib/clear_page_64.S | 13 +++++++--
> 3 files changed, 67 insertions(+), 30 deletions(-)
>
> diff --git a/arch/x86/include/asm/string.h b/arch/x86/include/asm/string.h
> index c3c2c1914d65..17f6b5bfa8c1 100644
> --- a/arch/x86/include/asm/string.h
> +++ b/arch/x86/include/asm/string.h
> @@ -1,6 +1,52 @@
> /* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_X86_STRING_H
> +#define _ASM_X86_STRING_H
> +
> +#include <asm/asm.h>
> +#include <asm/alternative.h>
> +#include <asm/cpufeatures.h>
> +
> #ifdef CONFIG_X86_32
> # include <asm/string_32.h>
> #else
> # include <asm/string_64.h>
> #endif
> +
> +#ifdef CONFIG_X86_64
> +#define ALT_64(orig, alt, feat) ALTERNATIVE(orig, alt, feat)
> +#else
> +#define ALT_64(orig, alt, feat) orig "\n"
> +#endif
> +
> +static __always_inline void *__inline_memcpy(void *to, const void *from, size_t len)
> +{
> + void *ret = to;
> +
> + asm volatile("1:\n\t"
> + ALT_64("rep movsb",
> + "call rep_movs_alternative", ALT_NOT(X86_FEATURE_FSRM))
> + "2:\n\t"
> + _ASM_EXTABLE_UA(1b, 2b)
> + : "+c" (len), "+D" (to), "+S" (from), ASM_CALL_CONSTRAINT
> + : : "memory", _ASM_AX);
> +
> + return ret + len;
> +}
> +
> +static __always_inline void *__inline_memset(void *addr, int v, size_t len)
> +{
> + void *ret = addr;
> +
> + asm volatile("1:\n\t"
> + ALT_64("rep stosb",
> + "call rep_stos_alternative", ALT_NOT(X86_FEATURE_FSRM))
> + "2:\n\t"
> + _ASM_EXTABLE_UA(1b, 2b)
> + : "+c" (len), "+D" (addr), ASM_CALL_CONSTRAINT
> + : "a" ((uint8_t)v)

You shouldn't need the (uint8_t) cast (should that be (u8) anyway).
At best it doesn't matter, at worst it will add code to mask with 0xff.

> + : "memory", _ASM_SI, _ASM_DX);
> +
> + return ret + len;
> +}
> +
> +#endif /* _ASM_X86_STRING_H */
> diff --git a/arch/x86/include/asm/uaccess_64.h b/arch/x86/include/asm/uaccess_64.h
> index c8a5ae35c871..eb531e13e659 100644
> --- a/arch/x86/include/asm/uaccess_64.h
> +++ b/arch/x86/include/asm/uaccess_64.h
> @@ -13,6 +13,7 @@
> #include <asm/page.h>
> #include <asm/percpu.h>
> #include <asm/runtime-const.h>
> +#include <asm/string.h>
>
> /*
> * Virtual variable: there's no actual backing store for this,
> @@ -118,21 +119,12 @@ rep_movs_alternative(void *to, const void *from, unsigned len);
> static __always_inline __must_check unsigned long
> copy_user_generic(void *to, const void *from, unsigned long len)
> {
> + void *ret;
> +
> stac();
> - /*
> - * If CPU has FSRM feature, use 'rep movs'.
> - * Otherwise, use rep_movs_alternative.
> - */
> - asm volatile(
> - "1:\n\t"
> - ALTERNATIVE("rep movsb",
> - "call rep_movs_alternative", ALT_NOT(X86_FEATURE_FSRM))
> - "2:\n"
> - _ASM_EXTABLE_UA(1b, 2b)
> - :"+c" (len), "+D" (to), "+S" (from), ASM_CALL_CONSTRAINT
> - : : "memory", "rax");
> + ret = __inline_memcpy(to, from, len);
> clac();
> - return len;
> + return ret - to;
> }
>
> static __always_inline __must_check unsigned long
> @@ -178,25 +170,15 @@ rep_stos_alternative(void __user *addr, unsigned long len);
>
> static __always_inline __must_check unsigned long __clear_user(void __user *addr, unsigned long size)
> {
> + void *ptr = (__force void *)addr;
> + void *ret;
> +
> might_fault();
> stac();
> -
> - /*
> - * No memory constraint because it doesn't change any memory gcc
> - * knows about.
> - */
> - asm volatile(
> - "1:\n\t"
> - ALTERNATIVE("rep stosb",
> - "call rep_stos_alternative", ALT_NOT(X86_FEATURE_FSRS))
> - "2:\n"
> - _ASM_EXTABLE_UA(1b, 2b)
> - : "+c" (size), "+D" (addr), ASM_CALL_CONSTRAINT
> - : "a" (0));
> -
> + ret = __inline_memset(ptr, 0, size);
> clac();
>
> - return size;
> + return ret - ptr;
> }
>
> static __always_inline unsigned long clear_user(void __user *to, unsigned long n)
> diff --git a/arch/x86/lib/clear_page_64.S b/arch/x86/lib/clear_page_64.S
> index a508e4a8c66a..47b613690f84 100644
> --- a/arch/x86/lib/clear_page_64.S
> +++ b/arch/x86/lib/clear_page_64.S
> @@ -55,17 +55,26 @@ SYM_FUNC_END(clear_page_erms)
> EXPORT_SYMBOL_GPL(clear_page_erms)
>
> /*
> - * Default clear user-space.
> + * Default memset.
> * Input:
> * rdi destination
> + * rsi scratch
> * rcx count
> - * rax is zero
> + * al is value
> *
> * Output:
> * rcx: uncleared bytes or 0 if successful.
> + * rdx: clobbered
> */
> SYM_FUNC_START(rep_stos_alternative)
> ANNOTATE_NOENDBR
> +
> + movzbq %al, %rsi
> + movabs $0x0101010101010101, %rax
> +
> + /* RDX:RAX = RAX * RSI */
> + mulq %rsi

NAK - you can't do that here.
Neither %rsi nor %rdx can be trashed.
The function has a very explicit calling convention.

It is also almost certainly a waste of time.
Pretty much all the calls will be for a constant 0x00.
Rename it all memzero() ...

David

> +
> cmpq $64,%rcx
> jae .Lunrolled
>