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 <20220514080402.2650181-14-chenhuacai@loongson.cn> <1b5eb372-0e62-bdc8-ebdc-f49eab220d1b@xen0n.name>
In-Reply-To: <1b5eb372-0e62-bdc8-ebdc-f49eab220d1b@xen0n.name>
From:   Huacai Chen <chenhuacai@gmail.com>
Date:   Sat, 14 May 2022 22:39:19 +0800
Message-ID: <CAAhV-H44ESukAX5qhj8TQttOar7Kg5qU0_1grfLdLDAHbFAoQA@mail.gmail.com>
Subject: Re: [PATCH V10 13/22] LoongArch: Add system call support
To:     WANG Xuerui <kernel@xen0n.name>
Cc:     Huacai Chen <chenhuacai@loongson.cn>,
        Arnd Bergmann <arnd@arndb.de>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Airlie <airlied@linux.ie>,
        Jonathan Corbet <corbet@lwn.net>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        linux-arch <linux-arch@vger.kernel.org>,
        "open list:DOCUMENTATION" <linux-doc@vger.kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Xuefeng Li <lixuefeng@loongson.cn>,
        Yanteng Si <siyanteng@loongson.cn>,
        Guo Ren <guoren@kernel.org>,
        Jiaxun Yang <jiaxun.yang@flygoat.com>,
        Stephen Rothwell <sfr@canb.auug.org.au>
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Hi, Xuerui,

On Sat, May 14, 2022 at 9:53 PM WANG Xuerui <kernel@xen0n.name> wrote:
>
> Hi,
>
> On 5/14/22 16:03, Huacai Chen wrote:
> > Add system call support and related uaccess.h for LoongArch.
> >
> > Q: Why keep _ARCH_WANT_SYS_CLONE definition while there is clone3:
> > A: The latest glibc release (2.35) has some basic support for clone3 bu=
t
> >     it isn't complete. E.g., pthread_create() and spawni() have convert=
ed
> >     to use clone3 but fork() will still use clone. Moreover, some secco=
mp
> >     related applications can still not work perfectly with clone3. E.g.=
,
> >     Chromium sandbox cannot work at all and there is no solution for it=
,
> >     which is more terrible than the fork() story [1].
> >
> > [1] https://chromium-review.googlesource.com/c/chromium/src/+/2936184
> >
> > Signed-off-by: Huacai Chen <chenhuacai@loongson.cn>
> > ---
> >   arch/loongarch/include/asm/seccomp.h     |  20 ++
> >   arch/loongarch/include/asm/syscall.h     |  74 +++++++
> >   arch/loongarch/include/asm/uaccess.h     | 269 ++++++++++++++++++++++=
+
> >   arch/loongarch/include/asm/unistd.h      |  11 +
> >   arch/loongarch/include/uapi/asm/unistd.h |   5 +
> >   arch/loongarch/kernel/entry.S            |  89 ++++++++
> >   arch/loongarch/kernel/syscall.c          |  63 ++++++
> >   7 files changed, 531 insertions(+)
> >   create mode 100644 arch/loongarch/include/asm/seccomp.h
> >   create mode 100644 arch/loongarch/include/asm/syscall.h
> >   create mode 100644 arch/loongarch/include/asm/uaccess.h
> >   create mode 100644 arch/loongarch/include/asm/unistd.h
> >   create mode 100644 arch/loongarch/include/uapi/asm/unistd.h
> >   create mode 100644 arch/loongarch/kernel/entry.S
> >   create mode 100644 arch/loongarch/kernel/syscall.c
> >
> > diff --git a/arch/loongarch/include/asm/seccomp.h b/arch/loongarch/incl=
ude/asm/seccomp.h
> > new file mode 100644
> > index 000000000000..31d6ab42e43e
> > --- /dev/null
> > +++ b/arch/loongarch/include/asm/seccomp.h
> > @@ -0,0 +1,20 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +
> > +#ifndef _ASM_SECCOMP_H
> > +#define _ASM_SECCOMP_H
> > +
> > +#include <asm/unistd.h>
> > +
> > +#include <asm-generic/seccomp.h>
> > +
> > +#ifdef CONFIG_32BIT
> > +# define SECCOMP_ARCH_NATIVE         AUDIT_ARCH_LOONGARCH32
> > +# define SECCOMP_ARCH_NATIVE_NR              NR_syscalls
> > +# define SECCOMP_ARCH_NATIVE_NAME    "loongarch32"
> > +#else
> > +# define SECCOMP_ARCH_NATIVE         AUDIT_ARCH_LOONGARCH64
> > +# define SECCOMP_ARCH_NATIVE_NR              NR_syscalls
> > +# define SECCOMP_ARCH_NATIVE_NAME    "loongarch64"
> > +#endif
> > +
> > +#endif /* _ASM_SECCOMP_H */
> > diff --git a/arch/loongarch/include/asm/syscall.h b/arch/loongarch/incl=
ude/asm/syscall.h
> > new file mode 100644
> > index 000000000000..e286dc58476e
> > --- /dev/null
> > +++ b/arch/loongarch/include/asm/syscall.h
> > @@ -0,0 +1,74 @@
> > +/* SPDX-License-Identifier: GPL-2.0+ */
> > +/*
> > + * Author: Hanlu Li <lihanlu@loongson.cn>
> > + *         Huacai Chen <chenhuacai@loongson.cn>
> > + *
> > + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> > + */
> > +
> > +#ifndef __ASM_LOONGARCH_SYSCALL_H
> > +#define __ASM_LOONGARCH_SYSCALL_H
> > +
> > +#include <linux/compiler.h>
> > +#include <uapi/linux/audit.h>
> > +#include <linux/elf-em.h>
> > +#include <linux/kernel.h>
> > +#include <linux/sched.h>
> > +#include <linux/uaccess.h>
> > +#include <asm/ptrace.h>
> > +#include <asm/unistd.h>
> > +
> > +extern void *sys_call_table[];
> > +
> > +static inline long syscall_get_nr(struct task_struct *task,
> > +                               struct pt_regs *regs)
> > +{
> > +     return regs->regs[11];
> It may be better readability-wise to somehow change this 11 to something
> resembling A7, but it's okay as-is.
OK, keep it as is.

> > +}
> > +
> > +static inline void syscall_rollback(struct task_struct *task,
> > +                                 struct pt_regs *regs)
> > +{
> > +        regs->regs[4] =3D regs->orig_a0;
> > +}
> > +
> > +static inline long syscall_get_error(struct task_struct *task,
> > +                                  struct pt_regs *regs)
> > +{
> > +     unsigned long error =3D regs->regs[4];
> > +
> > +     return IS_ERR_VALUE(error) ? error : 0;
> > +}
> > +
> > +static inline long syscall_get_return_value(struct task_struct *task,
> > +                                         struct pt_regs *regs)
> > +{
> > +     return regs->regs[4];
> > +}
> > +
> > +static inline void syscall_set_return_value(struct task_struct *task,
> > +                                         struct pt_regs *regs,
> > +                                         int error, long val)
> > +{
> > +     regs->regs[4] =3D (long) error ? error : val;
> > +}
> > +
> > +static inline void syscall_get_arguments(struct task_struct *task,
> > +                                      struct pt_regs *regs,
> > +                                      unsigned long *args)
> > +{
> > +     args[0] =3D regs->orig_a0;
> > +     memcpy(&args[1], &regs->regs[5], 5 * sizeof(long));
> > +}
> > +
> > +static inline int syscall_get_arch(struct task_struct *task)
> > +{
> > +     return AUDIT_ARCH_LOONGARCH64;
> > +}
> > +
> > +static inline bool arch_syscall_is_vdso_sigreturn(struct pt_regs *regs=
)
> > +{
> > +     return false;
> > +}
> > +
> > +#endif       /* __ASM_LOONGARCH_SYSCALL_H */
> > diff --git a/arch/loongarch/include/asm/uaccess.h b/arch/loongarch/incl=
ude/asm/uaccess.h
> > new file mode 100644
> > index 000000000000..217c6a3727b1
> > --- /dev/null
> > +++ b/arch/loongarch/include/asm/uaccess.h
> > @@ -0,0 +1,269 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> > + *
> > + * Derived from MIPS:
> > + * Copyright (C) 1996, 1997, 1998, 1999, 2000, 03, 04 by Ralf Baechle
> > + * Copyright (C) 1999, 2000 Silicon Graphics, Inc.
> > + * Copyright (C) 2007  Maciej W. Rozycki
> > + * Copyright (C) 2014, Imagination Technologies Ltd.
> > + */
> > +#ifndef _ASM_UACCESS_H
> > +#define _ASM_UACCESS_H
> > +
> > +#include <linux/kernel.h>
> > +#include <linux/string.h>
> > +#include <linux/extable.h>
> > +#include <asm/pgtable.h>
> > +#include <asm-generic/extable.h>
> > +#include <asm-generic/access_ok.h>
> > +
> > +extern u64 __ua_limit;
> > +
> > +#define __UA_ADDR    ".dword"
> > +#define __UA_LA              "la.abs"
> > +#define __UA_LIMIT   __ua_limit
> > +
> > +/*
> > + * get_user: - Get a simple variable from user space.
> > + * @x:        Variable to store result.
> > + * @ptr: Source address, in user space.
> > + *
> > + * Context: User context only. This function may sleep if pagefaults a=
re
> > + *          enabled.
> > + *
> > + * This macro copies a single simple variable from user space to kerne=
l
> > + * space.  It supports simple types like char and int, but not larger
> > + * data types like structures or arrays.
> > + *
> > + * @ptr must have pointer-to-simple-variable type, and the result of
> > + * dereferencing @ptr must be assignable to @x without a cast.
> > + *
> > + * Returns zero on success, or -EFAULT on error.
> > + * On error, the variable @x is set to zero.
> > + */
> > +#define get_user(x, ptr) \
> > +({                                                                   \
> > +     const __typeof__(*(ptr)) __user *__p =3D (ptr);                  =
 \
> > +                                                                     \
> > +     might_fault();                                                  \
> > +     access_ok(__p, sizeof(*__p)) ? __get_user((x), __p) :           \
> > +                                    ((x) =3D 0, -EFAULT);             =
 \
> > +})
> > +
> > +/*
> > + * put_user: - Write a simple value into user space.
> > + * @x:        Value to copy to user space.
> > + * @ptr: Destination address, in user space.
> > + *
> > + * Context: User context only. This function may sleep if pagefaults a=
re
> > + *          enabled.
> > + *
> > + * This macro copies a single simple value from kernel space to user
> > + * space.  It supports simple types like char and int, but not larger
> > + * data types like structures or arrays.
> > + *
> > + * @ptr must have pointer-to-simple-variable type, and @x must be assi=
gnable
> > + * to the result of dereferencing @ptr.
> > + *
> > + * Returns zero on success, or -EFAULT on error.
> > + */
> > +#define put_user(x, ptr) \
> > +({                                                                   \
> > +     __typeof__(*(ptr)) __user *__p =3D (ptr);                        =
 \
> > +                                                                     \
> > +     might_fault();                                                  \
> > +     access_ok(__p, sizeof(*__p)) ? __put_user((x), __p) : -EFAULT;  \
> > +})
> > +
> > +/*
> > + * __get_user: - Get a simple variable from user space, with less chec=
king.
> > + * @x:        Variable to store result.
> > + * @ptr: Source address, in user space.
> > + *
> > + * Context: User context only. This function may sleep if pagefaults a=
re
> > + *          enabled.
> > + *
> > + * This macro copies a single simple variable from user space to kerne=
l
> > + * space.  It supports simple types like char and int, but not larger
> > + * data types like structures or arrays.
> > + *
> > + * @ptr must have pointer-to-simple-variable type, and the result of
> > + * dereferencing @ptr must be assignable to @x without a cast.
> > + *
> > + * Caller must check the pointer with access_ok() before calling this
> > + * function.
> > + *
> > + * Returns zero on success, or -EFAULT on error.
> > + * On error, the variable @x is set to zero.
> > + */
> > +#define __get_user(x, ptr) \
> > +({                                                                   \
> > +     int __gu_err =3D 0;                                              =
 \
> > +                                                                     \
> > +     __chk_user_ptr(ptr);                                            \
> > +     __get_user_common((x), sizeof(*(ptr)), ptr);                    \
> > +     __gu_err;                                                       \
> > +})
> > +
> > +/*
> > + * __put_user: - Write a simple value into user space, with less check=
ing.
> > + * @x:        Value to copy to user space.
> > + * @ptr: Destination address, in user space.
> > + *
> > + * Context: User context only. This function may sleep if pagefaults a=
re
> > + *          enabled.
> > + *
> > + * This macro copies a single simple value from kernel space to user
> > + * space.  It supports simple types like char and int, but not larger
> > + * data types like structures or arrays.
> > + *
> > + * @ptr must have pointer-to-simple-variable type, and @x must be assi=
gnable
> > + * to the result of dereferencing @ptr.
> > + *
> > + * Caller must check the pointer with access_ok() before calling this
> > + * function.
> > + *
> > + * Returns zero on success, or -EFAULT on error.
> > + */
> > +#define __put_user(x, ptr) \
> > +({                                                                   \
> > +     int __pu_err =3D 0;                                              =
 \
> > +     __typeof__(*(ptr)) __pu_val;                                    \
> > +                                                                     \
> > +     __pu_val =3D (x);                                                =
 \
> > +     __chk_user_ptr(ptr);                                            \
> > +     __put_user_common(ptr, sizeof(*(ptr)));                         \
> > +     __pu_err;                                                       \
> > +})
> > +
> > +struct __large_struct { unsigned long buf[100]; };
> > +#define __m(x) (*(struct __large_struct __user *)(x))
> > +
> > +#define __get_user_common(val, size, ptr)                            \
> > +do {                                                                 \
> > +     switch (size) {                                                 \
> > +     case 1: __get_data_asm(val, "ld.b", ptr); break;                \
> > +     case 2: __get_data_asm(val, "ld.h", ptr); break;                \
> > +     case 4: __get_data_asm(val, "ld.w", ptr); break;                \
> > +     case 8: __get_data_asm(val, "ld.d", ptr); break;                \
> > +     default: BUILD_BUG(); break;                                    \
> > +     }                                                               \
> > +} while (0)
> > +
> > +#define __get_kernel_common(val, size, ptr) __get_user_common(val, siz=
e, ptr)
> > +
> > +#define __get_data_asm(val, insn, ptr)                                =
       \
> > +{                                                                    \
> > +     long __gu_tmp;                                                  \
> > +                                                                     \
> > +     __asm__ __volatile__(                                           \
> > +     "1:     " insn "        %1, %2                          \n"     \
> > +     "2:                                                     \n"     \
> > +     "       .section .fixup,\"ax\"                          \n"     \
> > +     "3:     li.w    %0, %3                                  \n"     \
> > +     "       or      %1, $r0, $r0                            \n"     \
> > +     "       b       2b                                      \n"     \
> > +     "       .previous                                       \n"     \
> > +     "       .section __ex_table,\"a\"                       \n"     \
> > +     "       "__UA_ADDR "\t1b, 3b                            \n"     \
> > +     "       .previous                                       \n"     \
> > +     : "+r" (__gu_err), "=3Dr" (__gu_tmp)                             =
 \
> > +     : "m" (__m(ptr)), "i" (-EFAULT));                               \
> > +                                                                     \
> > +     (val) =3D (__typeof__(*(ptr))) __gu_tmp;                         =
 \
> > +}
> > +
> > +#define __put_user_common(ptr, size)                                 \
> > +do {                                                                 \
> > +     switch (size) {                                                 \
> > +     case 1: __put_data_asm("st.b", ptr); break;                     \
> > +     case 2: __put_data_asm("st.h", ptr); break;                     \
> > +     case 4: __put_data_asm("st.w", ptr); break;                     \
> > +     case 8: __put_data_asm("st.d", ptr); break;                     \
> > +     default: BUILD_BUG(); break;                                    \
> > +     }                                                               \
> > +} while (0)
> > +
> > +#define __put_kernel_common(ptr, size) __put_user_common(ptr, size)
> > +
> > +#define __put_data_asm(insn, ptr)                                    \
> > +{                                                                    \
> > +     __asm__ __volatile__(                                           \
> > +     "1:     " insn "        %z2, %1         # __put_user_asm\n"     \
> > +     "2:                                                     \n"     \
> > +     "       .section        .fixup,\"ax\"                   \n"     \
> > +     "3:     li.w    %0, %3                                  \n"     \
> > +     "       b       2b                                      \n"     \
> > +     "       .previous                                       \n"     \
> > +     "       .section        __ex_table,\"a\"                \n"     \
> > +     "       " __UA_ADDR "   1b, 3b                          \n"     \
> > +     "       .previous                                       \n"     \
> > +     : "+r" (__pu_err), "=3Dm" (__m(ptr))                             =
 \
> > +     : "Jr" (__pu_val), "i" (-EFAULT));                              \
> > +}
> > +
> > +#define __get_kernel_nofault(dst, src, type, err_label)               =
       \
> > +do {                                                                 \
> > +     int __gu_err =3D 0;                                              =
 \
> > +                                                                     \
> > +     __get_kernel_common(*((type *)(dst)), sizeof(type),             \
> > +                         (__force type *)(src));                     \
> > +     if (unlikely(__gu_err))                                         \
> > +             goto err_label;                                         \
> > +} while (0)
> > +
> > +#define __put_kernel_nofault(dst, src, type, err_label)               =
       \
> > +do {                                                                 \
> > +     type __pu_val;                                                  \
> > +     int __pu_err =3D 0;                                              =
 \
> > +                                                                     \
> > +     __pu_val =3D *(__force type *)(src);                             =
 \
> > +     __put_kernel_common(((type *)(dst)), sizeof(type));             \
> > +     if (unlikely(__pu_err))                                         \
> > +             goto err_label;                                         \
> > +} while (0)
> > +
> > +extern unsigned long __copy_user(void *to, const void *from, __kernel_=
size_t n);
> > +
> > +static inline unsigned long __must_check
> > +raw_copy_from_user(void *to, const void __user *from, unsigned long n)
> > +{
> > +     return __copy_user(to, from, n);
> > +}
> > +
> > +static inline unsigned long __must_check
> > +raw_copy_to_user(void __user *to, const void *from, unsigned long n)
> > +{
> > +     return __copy_user(to, from, n);
> > +}
> > +
> > +#define INLINE_COPY_FROM_USER
> > +#define INLINE_COPY_TO_USER
> > +
> > +/*
> > + * __clear_user: - Zero a block of memory in user space, with less che=
cking.
> > + * @addr: Destination address, in user space.
> > + * @size: Number of bytes to zero.
> > + *
> > + * Zero a block of memory in user space.  Caller must check
> > + * the specified block with access_ok() before calling this function.
> > + *
> > + * Returns number of bytes that could not be cleared.
> > + * On success, this will be zero.
> > + */
> > +extern unsigned long __clear_user(void __user *addr, __kernel_size_t s=
ize);
> > +
> > +#define clear_user(addr, n)                                          \
> > +({                                                                   \
> > +     void __user *__cl_addr =3D (addr);                               =
 \
> > +     unsigned long __cl_size =3D (n);                                 =
 \
> > +     if (__cl_size && access_ok(__cl_addr, __cl_size))               \
> > +             __cl_size =3D __clear_user(__cl_addr, __cl_size);        =
 \
> > +     __cl_size;                                                      \
> > +})
> > +
> > +extern long strncpy_from_user(char *to, const char __user *from, long =
n);
> > +extern long strnlen_user(const char __user *str, long n);
> > +
> > +#endif /* _ASM_UACCESS_H */
> > diff --git a/arch/loongarch/include/asm/unistd.h b/arch/loongarch/inclu=
de/asm/unistd.h
> > new file mode 100644
> > index 000000000000..cfddb0116a8c
> > --- /dev/null
> > +++ b/arch/loongarch/include/asm/unistd.h
> > @@ -0,0 +1,11 @@
> > +/* SPDX-License-Identifier: GPL-2.0+ */
> > +/*
> > + * Author: Hanlu Li <lihanlu@loongson.cn>
> > + *         Huacai Chen <chenhuacai@loongson.cn>
> > + *
> > + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> > + */
> > +
> > +#include <uapi/asm/unistd.h>
> > +
> > +#define NR_syscalls (__NR_syscalls)
> > diff --git a/arch/loongarch/include/uapi/asm/unistd.h b/arch/loongarch/=
include/uapi/asm/unistd.h
> > new file mode 100644
> > index 000000000000..fcb668984f03
> > --- /dev/null
> > +++ b/arch/loongarch/include/uapi/asm/unistd.h
> > @@ -0,0 +1,5 @@
> > +/* SPDX-License-Identifier: GPL-2.0 WITH Linux-syscall-note */
> > +#define __ARCH_WANT_SYS_CLONE
> > +#define __ARCH_WANT_SYS_CLONE3
> Having read the discussion in v9 regarding the sandbox scenario, I agree
> leaving clone alone would probably be the better choice. It's relieving
> seeing the stat cruft dropped... World rebuilds of new-world distros
> pending on the horizon. :-D
clone3() is not just a beautiful version of clone(), it has some new
features such as pidfd, so we need both clone() and clone3().

> > +
> > +#include <asm-generic/unistd.h>
> > diff --git a/arch/loongarch/kernel/entry.S b/arch/loongarch/kernel/entr=
y.S
> > new file mode 100644
> > index 000000000000..d5b3dbcf5425
> > --- /dev/null
> > +++ b/arch/loongarch/kernel/entry.S
> > @@ -0,0 +1,89 @@
> > +/* SPDX-License-Identifier: GPL-2.0 */
> > +/*
> > + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> > + *
> > + * Derived from MIPS:
> > + * Copyright (C) 1994 - 2000, 2001, 2003 Ralf Baechle
> > + * Copyright (C) 1999, 2000 Silicon Graphics, Inc.
> > + * Copyright (C) 2001 MIPS Technologies, Inc.
> > + */
> > +
> > +#include <asm/asm.h>
> > +#include <asm/asmmacro.h>
> > +#include <asm/loongarch.h>
> > +#include <asm/regdef.h>
> > +#include <asm/stackframe.h>
> > +#include <asm/thread_info.h>
> > +
> > +     .text
> > +     .cfi_sections   .debug_frame
> > +     .align  5
> > +SYM_FUNC_START(handle_syscall)
> > +     csrrd   t0, PERCPU_BASE_KS
> > +     la.abs  t1, kernelsp
> > +     add.d   t1, t1, t0
> > +     move    t2, sp
> > +     ld.d    sp, t1, 0
> > +
> > +     addi.d  sp, sp, -PT_SIZE
> > +     cfi_st  t2, PT_R3
> > +     cfi_rel_offset  sp, PT_R3
> > +     st.d    zero, sp, PT_R0
> > +     csrrd   t2, LOONGARCH_CSR_PRMD
> > +     st.d    t2, sp, PT_PRMD
> > +     csrrd   t2, LOONGARCH_CSR_CRMD
> > +     st.d    t2, sp, PT_CRMD
> > +     csrrd   t2, LOONGARCH_CSR_EUEN
> > +     st.d    t2, sp, PT_EUEN
> > +     csrrd   t2, LOONGARCH_CSR_ECFG
> > +     st.d    t2, sp, PT_ECFG
> > +     csrrd   t2, LOONGARCH_CSR_ESTAT
> > +     st.d    t2, sp, PT_ESTAT
> > +     cfi_st  ra, PT_R1
> > +     cfi_st  a0, PT_R4
> > +     cfi_st  a1, PT_R5
> > +     cfi_st  a2, PT_R6
> > +     cfi_st  a3, PT_R7
> > +     cfi_st  a4, PT_R8
> > +     cfi_st  a5, PT_R9
> > +     cfi_st  a6, PT_R10
> > +     cfi_st  a7, PT_R11
> > +     csrrd   ra, LOONGARCH_CSR_ERA
> > +     st.d    ra, sp, PT_ERA
> > +     cfi_rel_offset ra, PT_ERA
> > +
> > +     cfi_st  tp, PT_R2
> > +     cfi_st  u0, PT_R21
> > +     cfi_st  fp, PT_R22
> > +
> > +     SAVE_STATIC
> > +
> > +     move    u0, t0
> > +     li.d    tp, ~_THREAD_MASK
> > +     and     tp, tp, sp
> > +
> > +     move    a0, sp
> > +     bl      do_syscall
> > +
> > +     RESTORE_ALL_AND_RET
> > +SYM_FUNC_END(handle_syscall)
> > +
> > +SYM_CODE_START(ret_from_fork)
> > +     bl      schedule_tail           # a0 =3D struct task_struct *prev
> > +     move    a0, sp
> > +     bl      syscall_exit_to_user_mode
> > +     RESTORE_STATIC
> > +     RESTORE_SOME
> > +     RESTORE_SP_AND_RET
> > +SYM_CODE_END(ret_from_fork)
> > +
> > +SYM_CODE_START(ret_from_kernel_thread)
> > +     bl      schedule_tail           # a0 =3D struct task_struct *prev
> > +     move    a0, s1
> > +     jirl    ra, s0, 0
> > +     move    a0, sp
> > +     bl      syscall_exit_to_user_mode
> > +     RESTORE_STATIC
> > +     RESTORE_SOME
> > +     RESTORE_SP_AND_RET
> > +SYM_CODE_END(ret_from_kernel_thread)
> > diff --git a/arch/loongarch/kernel/syscall.c b/arch/loongarch/kernel/sy=
scall.c
> > new file mode 100644
> > index 000000000000..3fc4211db989
> > --- /dev/null
> > +++ b/arch/loongarch/kernel/syscall.c
> > @@ -0,0 +1,63 @@
> > +// SPDX-License-Identifier: GPL-2.0+
> > +/*
> > + * Author: Hanlu Li <lihanlu@loongson.cn>
> > + *         Huacai Chen <chenhuacai@loongson.cn>
> > + *
> > + * Copyright (C) 2020-2022 Loongson Technology Corporation Limited
> > + */
> > +#include <linux/capability.h>
> > +#include <linux/entry-common.h>
> > +#include <linux/errno.h>
> > +#include <linux/linkage.h>
> > +#include <linux/syscalls.h>
> > +#include <linux/unistd.h>
> > +
> > +#include <asm/asm.h>
> > +#include <asm/signal.h>
> > +#include <asm/switch_to.h>
> > +#include <asm-generic/syscalls.h>
> > +
> > +#undef __SYSCALL
> > +#define __SYSCALL(nr, call)  [nr] =3D (call),
> > +
> > +SYSCALL_DEFINE6(mmap, unsigned long, addr, unsigned long, len, unsigne=
d long,
> > +             prot, unsigned long, flags, unsigned long, fd, off_t, off=
set)
> > +{
> > +     if (offset & ~PAGE_MASK)
> > +             return -EINVAL;
> > +
> > +     return ksys_mmap_pgoff(addr, len, prot, flags, fd, offset >> PAGE=
_SHIFT);
> > +}
> > +
> > +void *sys_call_table[__NR_syscalls] =3D {
> > +     [0 ... __NR_syscalls - 1] =3D sys_ni_syscall,
> > +#include <asm/unistd.h>
> > +};
> > +
> > +typedef long (*sys_call_fn)(unsigned long, unsigned long,
> > +     unsigned long, unsigned long, unsigned long, unsigned long);
> > +
> > +void noinstr do_syscall(struct pt_regs *regs)
> > +{
> > +     unsigned long nr;
> > +     sys_call_fn syscall_fn;
> > +
> > +     nr =3D regs->regs[11];
> > +     /* Set for syscall restarting */
> > +     if (nr < NR_syscalls)
> > +             regs->regs[0] =3D nr + 1;
> > +
> > +     regs->csr_era +=3D 4;
> > +     regs->orig_a0 =3D regs->regs[4];
> > +     regs->regs[4] =3D -ENOSYS;
> > +
> > +     nr =3D syscall_enter_from_user_mode(regs, nr);
> > +
> > +     if (nr < NR_syscalls) {
>
> I saw in the manual that there's the BADI (=E5=87=BA=E9=94=99=E6=8C=87=E4=
=BB=A4) CSR that records the
> faulting instruction word for exceptions other than INT, GCHC and MERR;
> this implies we can read the trapping syscall insn word from this CSR,
> to inspect its immediate field, so we can reserve non-zero immediates on
> the syscall insn for future use.
>
> This would take 3 insns to complete: csrrd, bitfield extraction, and
> bail if non-zero; this should be an acceptable tradeoff for a lot of
> extensibility and scalability?
This need much efforts, we can optimize it in future.

Huacai
>
> > +             syscall_fn =3D sys_call_table[nr];
> > +             regs->regs[4] =3D syscall_fn(regs->orig_a0, regs->regs[5]=
, regs->regs[6],
> > +                                        regs->regs[7], regs->regs[8], =
regs->regs[9]);
> > +     }
> > +
> > +     syscall_exit_to_user_mode(regs);
> > +}