Re: [PATCH v2 3/4] loongarch: entry: Migrate ret_from_fork() to C

[Charlie Jenkins <charlie@rivosinc.com>]

On Fri, Jan 24, 2025 at 05:05:21PM +0800, Huacai Chen wrote:
> Hi, Charlie,
> 
> On Fri, Jan 24, 2025 at 3:15 AM Charlie Jenkins <charlie@rivosinc.com> wrote:
> >
> > Loongarch is the only architecture that calls
> We usually use "LoongArch" instead of "loongarch" or "Loongarch".
> 
> > syscall_exit_to_user_mode() from asm. Move the call into C so that this
> > function can be inlined across all architectures.
> >
> > Signed-off-by: Charlie Jenkins <charlie@rivosinc.com>
> > ---
> >  arch/loongarch/include/asm/asm-prototypes.h |  5 +++++
> >  arch/loongarch/include/asm/switch_to.h      |  8 +++++++
> >  arch/loongarch/kernel/entry.S               | 22 +++++++++----------
> >  arch/loongarch/kernel/process.c             | 34 ++++++++++++++++++++++++-----
> >  4 files changed, 51 insertions(+), 18 deletions(-)
> >
> > diff --git a/arch/loongarch/include/asm/asm-prototypes.h b/arch/loongarch/include/asm/asm-prototypes.h
> > index 51f224bcfc654228ae423e9a066b25b35102a5b9..0195d4309fd29f94664d5f34247198c769033b1b 100644
> > --- a/arch/loongarch/include/asm/asm-prototypes.h
> > +++ b/arch/loongarch/include/asm/asm-prototypes.h
> > @@ -12,3 +12,8 @@ __int128_t __ashlti3(__int128_t a, int b);
> >  __int128_t __ashrti3(__int128_t a, int b);
> >  __int128_t __lshrti3(__int128_t a, int b);
> >  #endif
> > +
> > +asmlinkage void noinstr __no_stack_protector ret_from_kernel_thread(struct task_struct *prev,
> > +                                                                   struct pt_regs *regs,
> > +                                                                   int (*fn)(void *),
> > +                                                                   void *fn_arg);
> It is a little strange that we only need to declare
> ret_from_kernel_thread() but not ret_from_fork().

Just an oversight by me, thank you for pointing that out.

> 
> > diff --git a/arch/loongarch/include/asm/switch_to.h b/arch/loongarch/include/asm/switch_to.h
> > index 5b225aff3ba21aa06d0713bc8e73e1b941389630..a1c5576f1fd145670e13038bec6dd390486099ab 100644
> > --- a/arch/loongarch/include/asm/switch_to.h
> > +++ b/arch/loongarch/include/asm/switch_to.h
> > @@ -26,6 +26,14 @@ extern asmlinkage struct task_struct *__switch_to(struct task_struct *prev,
> >                         struct task_struct *next, struct thread_info *next_ti,
> >                         void *sched_ra, void *sched_cfa);
> >
> > +void noinstr __no_stack_protector ret_from_kernel_thread(struct task_struct *prev,
> > +                                                        struct pt_regs *regs,
> > +                                                        int (*fn)(void *),
> > +                                                        void *fn_arg);
> > +
> > +void noinstr __no_stack_protector ret_from_fork(struct task_struct *prev,
> > +                                               struct pt_regs *regs);
> > +
> I prefer alpha-betical order, which means put ret_from_fork() before
> ret_from_kernel_thread().

Makes sense!

> 
> >  /*
> >   * For newly created kernel threads switch_to() will return to
> >   * ret_from_kernel_thread, newly created user threads to ret_from_fork.
> > diff --git a/arch/loongarch/kernel/entry.S b/arch/loongarch/kernel/entry.S
> > index 48e7e34e355e83eae8165957ba2eac05a8bf17df..2abc29e573810e000f2fef4646ddca0dbb80eabe 100644
> > --- a/arch/loongarch/kernel/entry.S
> > +++ b/arch/loongarch/kernel/entry.S
> > @@ -77,24 +77,22 @@ SYM_CODE_START(handle_syscall)
> >  SYM_CODE_END(handle_syscall)
> >  _ASM_NOKPROBE(handle_syscall)
> >
> > -SYM_CODE_START(ret_from_fork)
> > +SYM_CODE_START(ret_from_fork_asm)
> >         UNWIND_HINT_REGS
> > -       bl              schedule_tail           # a0 = struct task_struct *prev
> > -       move            a0, sp
> > -       bl              syscall_exit_to_user_mode
> > +       move            a1, sp
> > +       bl              ret_from_fork
> >         RESTORE_STATIC
> >         RESTORE_SOME
> >         RESTORE_SP_AND_RET
> > -SYM_CODE_END(ret_from_fork)
> > +SYM_CODE_END(ret_from_fork_asm)
> >
> > -SYM_CODE_START(ret_from_kernel_thread)
> > +SYM_CODE_START(ret_from_kernel_thread_asm)
> >         UNWIND_HINT_REGS
> > -       bl              schedule_tail           # a0 = struct task_struct *prev
> > -       move            a0, s1
> > -       jirl            ra, s0, 0
> > -       move            a0, sp
> > -       bl              syscall_exit_to_user_mode
> > +       move            a1, sp
> > +       move            a2, s0
> > +       move            a3, s1
> > +       bl              ret_from_kernel_thread
> >         RESTORE_STATIC
> >         RESTORE_SOME
> >         RESTORE_SP_AND_RET
> > -SYM_CODE_END(ret_from_kernel_thread)
> > +SYM_CODE_END(ret_from_kernel_thread_asm)
> > diff --git a/arch/loongarch/kernel/process.c b/arch/loongarch/kernel/process.c
> > index 6e58f65455c7ca3eae2e88ed852c8655a6701e5c..16cc949fe43443d70f1d865ce04595c2d8c1615b 100644
> > --- a/arch/loongarch/kernel/process.c
> > +++ b/arch/loongarch/kernel/process.c
> > @@ -14,6 +14,7 @@
> >  #include <linux/init.h>
> >  #include <linux/kernel.h>
> >  #include <linux/errno.h>
> > +#include <linux/entry-common.h>
> >  #include <linux/sched.h>
> >  #include <linux/sched/debug.h>
> >  #include <linux/sched/task.h>
> > @@ -33,6 +34,7 @@
> >  #include <linux/prctl.h>
> >  #include <linux/nmi.h>
> >
> > +#include <asm/asm-prototypes.h>
> >  #include <asm/asm.h>
> >  #include <asm/bootinfo.h>
> >  #include <asm/cpu.h>
> > @@ -47,6 +49,7 @@
> >  #include <asm/pgtable.h>
> >  #include <asm/processor.h>
> >  #include <asm/reg.h>
> > +#include <asm/switch_to.h>
> >  #include <asm/unwind.h>
> >  #include <asm/vdso.h>
> >
> > @@ -63,8 +66,9 @@ EXPORT_SYMBOL(__stack_chk_guard);
> >  unsigned long boot_option_idle_override = IDLE_NO_OVERRIDE;
> >  EXPORT_SYMBOL(boot_option_idle_override);
> >
> > -asmlinkage void ret_from_fork(void);
> > -asmlinkage void ret_from_kernel_thread(void);
> > +asmlinkage void restore_and_ret(void);
> > +asmlinkage void ret_from_fork_asm(void);
> > +asmlinkage void ret_from_kernel_thread_asm(void);
> >
> >  void start_thread(struct pt_regs *regs, unsigned long pc, unsigned long sp)
> >  {
> > @@ -138,6 +142,24 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
> >         return 0;
> >  }
> >
> > +asmlinkage void noinstr __no_stack_protector ret_from_kernel_thread(struct task_struct *prev,
> > +                                                                   struct pt_regs *regs,
> > +                                                                   int (*fn)(void *),
> > +                                                                   void *fn_arg)
> > +{
> > +       schedule_tail(prev);
> > +
> > +       fn(fn_arg);
> > +
> The two blank lines can be removed, and again, I prefer alpha-betical
> order, which means put ret_from_fork() before
> ret_from_kernel_thread().

Will update in the next version.

- Charlie

> 
> Huacai
> 
> > +       syscall_exit_to_user_mode(regs);
> > +}
> > +
> > +void noinstr __no_stack_protector ret_from_fork(struct task_struct *prev, struct pt_regs *regs)
> > +{
> > +       schedule_tail(prev);
> > +       syscall_exit_to_user_mode(regs);
> > +}
> > +
> >  /*
> >   * Copy architecture-specific thread state
> >   */
> > @@ -165,8 +187,8 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
> >                 p->thread.reg03 = childksp;
> >                 p->thread.reg23 = (unsigned long)args->fn;
> >                 p->thread.reg24 = (unsigned long)args->fn_arg;
> > -               p->thread.reg01 = (unsigned long)ret_from_kernel_thread;
> > -               p->thread.sched_ra = (unsigned long)ret_from_kernel_thread;
> > +               p->thread.reg01 = (unsigned long)ret_from_kernel_thread_asm;
> > +               p->thread.sched_ra = (unsigned long)ret_from_kernel_thread_asm;
> >                 memset(childregs, 0, sizeof(struct pt_regs));
> >                 childregs->csr_euen = p->thread.csr_euen;
> >                 childregs->csr_crmd = p->thread.csr_crmd;
> > @@ -182,8 +204,8 @@ int copy_thread(struct task_struct *p, const struct kernel_clone_args *args)
> >                 childregs->regs[3] = usp;
> >
> >         p->thread.reg03 = (unsigned long) childregs;
> > -       p->thread.reg01 = (unsigned long) ret_from_fork;
> > -       p->thread.sched_ra = (unsigned long) ret_from_fork;
> > +       p->thread.reg01 = (unsigned long) ret_from_fork_asm;
> > +       p->thread.sched_ra = (unsigned long) ret_from_fork_asm;
> >
> >         /*
> >          * New tasks lose permission to use the fpu. This accelerates context
> >
> > --
> > 2.43.0
> >

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