linux-kernel - Re: [PATCH -next 4/7] riscv: syscall: Don't clobber s0 when syscall

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Date:   Wed, 21 Sep 2022 09:30:46 +0800
From:   Guo Ren <guoren@...nel.org>
To:     Chen Zhongjin <chenzhongjin@...wei.com>
Cc:     linux-kernel@...r.kernel.org, linux-riscv@...ts.infradead.org,
        linux-perf-users@...r.kernel.org, paul.walmsley@...ive.com,
        palmer@...belt.com, aou@...s.berkeley.edu, peterz@...radead.org,
        mingo@...hat.com, acme@...nel.org, mark.rutland@....com,
        alexander.shishkin@...ux.intel.com, namhyung@...nel.org,
        jolsa@...nel.org, nsaenzju@...hat.com, frederic@...nel.org,
        changbin.du@...el.com, vincent.chen@...ive.com, ardb@...nel.org,
        mhiramat@...nel.org, rostedt@...dmis.org, keescook@...omium.org,
        catalin.marinas@....com
Subject: Re: [PATCH -next 4/7] riscv: syscall: Don't clobber s0 when syscall

On Tue, Sep 20, 2022 at 11:15 PM Chen Zhongjin <chenzhongjin@...wei.com> wrote:
>
> syscall uses s0 to load address of sys_call_table.
>
> Since now we uses s0 to save pt_regs for unwinding, clobber
> s0 can make unwinder treat s0 as pt_regs address. Use s1 for
> this job.
This patch does not make sense. Why couldn't we use s1 for pt_regs?

No need to modify the entry.S here.

>
> Signed-off-by: Chen Zhongjin <chenzhongjin@...wei.com>
> ---
>  arch/riscv/kernel/entry.S | 20 ++++++++++----------
>  1 file changed, 10 insertions(+), 10 deletions(-)
>
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index ecb15c7430b4..a3b14a649782 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -205,33 +205,33 @@ handle_syscall:
>  check_syscall_nr:
>         /* Check to make sure we don't jump to a bogus syscall number. */
>         li t0, __NR_syscalls
> -       la s0, sys_ni_syscall
> +       la s1, sys_ni_syscall
>         /*
>          * Syscall number held in a7.
>          * If syscall number is above allowed value, redirect to ni_syscall.
>          */
>         bgeu a7, t0, 3f
>  #ifdef CONFIG_COMPAT
> -       REG_L s0, PT_STATUS(sp)
> -       srli s0, s0, SR_UXL_SHIFT
> -       andi s0, s0, (SR_UXL >> SR_UXL_SHIFT)
> +       REG_L s1, PT_STATUS(sp)
> +       srli s1, s1, SR_UXL_SHIFT
> +       andi s1, s1, (SR_UXL >> SR_UXL_SHIFT)
>         li t0, (SR_UXL_32 >> SR_UXL_SHIFT)
> -       sub t0, s0, t0
> +       sub t0, s1, t0
>         bnez t0, 1f
>
>         /* Call compat_syscall */
> -       la s0, compat_sys_call_table
> +       la s1, compat_sys_call_table
>         j 2f
>  1:
>  #endif
>         /* Call syscall */
> -       la s0, sys_call_table
> +       la s1, sys_call_table
>  2:
>         slli t0, a7, RISCV_LGPTR
> -       add s0, s0, t0
> -       REG_L s0, 0(s0)
> +       add s1, s1, t0
> +       REG_L s1, 0(s1)
>  3:
> -       jalr s0
> +       jalr s1
>
>  ret_from_syscall:
>         /* Set user a0 to kernel a0 */
> --
> 2.17.1
>


-- 
Best Regards
 Guo Ren