Date:   Sun, 26 Jul 2020 14:33:20 -0400
From:   Brian Gerst <brgerst@...il.com>
To:     Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Cc:     linux-tip-commits@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>, x86 <x86@...nel.org>
Subject: Re: [tip: x86/entry] x86/entry: Consolidate 32/64 bit syscall entry

On Fri, Jul 24, 2020 at 4:14 PM tip-bot2 for Thomas Gleixner
<tip-bot2@...utronix.de> wrote:
>
> The following commit has been merged into the x86/entry branch of tip:
>
> Commit-ID:     0b085e68f4072024ecaa3889aeeaab5f6c8eba5c
> Gitweb:        https://git.kernel.org/tip/0b085e68f4072024ecaa3889aeeaab5f6c8eba5c
> Author:        Thomas Gleixner <tglx@...utronix.de>
> AuthorDate:    Thu, 23 Jul 2020 00:00:01 +02:00
> Committer:     Thomas Gleixner <tglx@...utronix.de>
> CommitterDate: Fri, 24 Jul 2020 15:04:58 +02:00
>
> x86/entry: Consolidate 32/64 bit syscall entry
>
> 64bit and 32bit entry code have the same open coded syscall entry handling
> after the bitwidth specific bits.
>
> Move it to a helper function and share the code.
>
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Link: https://lkml.kernel.org/r/20200722220520.051234096@linutronix.de
>
>
> ---
>  arch/x86/entry/common.c | 93 +++++++++++++++++-----------------------
>  1 file changed, 41 insertions(+), 52 deletions(-)
>
> diff --git a/arch/x86/entry/common.c b/arch/x86/entry/common.c
> index ab6cb86..68d5c86 100644
> --- a/arch/x86/entry/common.c
> +++ b/arch/x86/entry/common.c
> @@ -366,8 +366,7 @@ __visible noinstr void syscall_return_slowpath(struct pt_regs *regs)
>         exit_to_user_mode();
>  }
>
> -#ifdef CONFIG_X86_64
> -__visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
> +static noinstr long syscall_enter(struct pt_regs *regs, unsigned long nr)
>  {
>         struct thread_info *ti;
>
> @@ -379,6 +378,16 @@ __visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
>         if (READ_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY)
>                 nr = syscall_trace_enter(regs);
>
> +       instrumentation_end();
> +       return nr;
> +}
> +
> +#ifdef CONFIG_X86_64
> +__visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
> +{
> +       nr = syscall_enter(regs, nr);
> +
> +       instrumentation_begin();
>         if (likely(nr < NR_syscalls)) {
>                 nr = array_index_nospec(nr, NR_syscalls);
>                 regs->ax = sys_call_table[nr](regs);
> @@ -390,64 +399,53 @@ __visible noinstr void do_syscall_64(unsigned long nr, struct pt_regs *regs)
>                 regs->ax = x32_sys_call_table[nr](regs);
>  #endif
>         }
> -       __syscall_return_slowpath(regs);
> -
>         instrumentation_end();
> -       exit_to_user_mode();
> +       syscall_return_slowpath(regs);
>  }
>  #endif
>
>  #if defined(CONFIG_X86_32) || defined(CONFIG_IA32_EMULATION)
> +static __always_inline unsigned int syscall_32_enter(struct pt_regs *regs)
> +{
> +       if (IS_ENABLED(CONFIG_IA32_EMULATION))
> +               current_thread_info()->status |= TS_COMPAT;
> +       /*
> +        * Subtlety here: if ptrace pokes something larger than 2^32-1 into
> +        * orig_ax, the unsigned int return value truncates it.  This may
> +        * or may not be necessary, but it matches the old asm behavior.
> +        */
> +       return syscall_enter(regs, (unsigned int)regs->orig_ax);
> +}
> +
>  /*
> - * Does a 32-bit syscall.  Called with IRQs on in CONTEXT_KERNEL.  Does
> - * all entry and exit work and returns with IRQs off.  This function is
> - * extremely hot in workloads that use it, and it's usually called from
> - * do_fast_syscall_32, so forcibly inline it to improve performance.
> + * Invoke a 32-bit syscall.  Called with IRQs on in CONTEXT_KERNEL.
>   */
> -static void do_syscall_32_irqs_on(struct pt_regs *regs)
> +static __always_inline void do_syscall_32_irqs_on(struct pt_regs *regs,
> +                                                 unsigned int nr)
>  {
> -       struct thread_info *ti = current_thread_info();
> -       unsigned int nr = (unsigned int)regs->orig_ax;
> -
> -#ifdef CONFIG_IA32_EMULATION
> -       ti->status |= TS_COMPAT;
> -#endif
> -
> -       if (READ_ONCE(ti->flags) & _TIF_WORK_SYSCALL_ENTRY) {
> -               /*
> -                * Subtlety here: if ptrace pokes something larger than
> -                * 2^32-1 into orig_ax, this truncates it.  This may or
> -                * may not be necessary, but it matches the old asm
> -                * behavior.
> -                */
> -               nr = syscall_trace_enter(regs);
> -       }
> -
>         if (likely(nr < IA32_NR_syscalls)) {
> +               instrumentation_begin();
>                 nr = array_index_nospec(nr, IA32_NR_syscalls);
>                 regs->ax = ia32_sys_call_table[nr](regs);
> +               instrumentation_end();
>         }
> -
> -       __syscall_return_slowpath(regs);
>  }
>
>  /* Handles int $0x80 */
>  __visible noinstr void do_int80_syscall_32(struct pt_regs *regs)
>  {
> -       enter_from_user_mode(regs);
> -       instrumentation_begin();
> +       unsigned int nr = syscall_32_enter(regs);
>
> -       local_irq_enable();
> -       do_syscall_32_irqs_on(regs);
> -
> -       instrumentation_end();
> -       exit_to_user_mode();
> +       do_syscall_32_irqs_on(regs, nr);
> +       syscall_return_slowpath(regs);
>  }
>
> -static bool __do_fast_syscall_32(struct pt_regs *regs)
> +static noinstr bool __do_fast_syscall_32(struct pt_regs *regs)

Can __do_fast_syscall_32() be merged back into do_fast_syscall_32()
now that both are marked noinstr?

--
Brian Gerst

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