linux-kernel - Re: [PATCH] LoongArch: Remove some save operations in SAVE_SOME and handle

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Date:   Thu, 29 Sep 2022 09:35:01 +0800
From:   Jinyang He <hejinyang@...ngson.cn>
To:     Tiezhu Yang <yangtiezhu@...ngson.cn>,
        Huacai Chen <chenhuacai@...nel.org>
Cc:     loongarch@...ts.linux.dev, linux-kernel@...r.kernel.org
Subject: Re: [PATCH] LoongArch: Remove some save operations in SAVE_SOME and
 handle_syscall

On 2022/9/29 上午7:59, Tiezhu Yang wrote:

> No need to save the control and status registers CRMD, EUEN, ECFG
> and ESTAT in SAVE_SOME and handle_syscall as they are not restored
> later in RESTORE_SOME.
>
> Signed-off-by: Tiezhu Yang <yangtiezhu@...ngson.cn>
> ---
>   arch/loongarch/include/asm/stackframe.h | 9 ---------
>   arch/loongarch/kernel/entry.S           | 8 --------
>   2 files changed, 17 deletions(-)
>
> diff --git a/arch/loongarch/include/asm/stackframe.h b/arch/loongarch/include/asm/stackframe.h
> index 4ca9530..733dc9e 100644
> --- a/arch/loongarch/include/asm/stackframe.h
> +++ b/arch/loongarch/include/asm/stackframe.h
> @@ -114,14 +114,6 @@
>   	LONG_S	zero, sp, PT_R0
>   	csrrd	t0, LOONGARCH_CSR_PRMD
>   	LONG_S	t0, sp, PT_PRMD
> -	csrrd	t0, LOONGARCH_CSR_CRMD
> -	LONG_S	t0, sp, PT_CRMD
> -	csrrd	t0, LOONGARCH_CSR_EUEN
> -	LONG_S  t0, sp, PT_EUEN
> -	csrrd	t0, LOONGARCH_CSR_ECFG
> -	LONG_S	t0, sp, PT_ECFG
> -	csrrd	t0, LOONGARCH_CSR_ESTAT
> -	PTR_S	t0, sp, PT_ESTAT
>   	cfi_st	ra, PT_R1, \docfi
>   	cfi_st	a0, PT_R4, \docfi
>   	cfi_st	a1, PT_R5, \docfi
> @@ -140,7 +132,6 @@
>   	cfi_st	fp, PT_R22, \docfi
>   
>   	/* Set thread_info if we're coming from user mode */
> -	csrrd	t0, LOONGARCH_CSR_PRMD
>   	andi	t0, t0, 0x3	/* extract pplv bit */
>   	beqz	t0, 9f

Hi, Tiezhu,


Have u tested debug functions like __show_regs? We discussed offline
about which registers should be saved in kretprobe lately. And I think
users use kprobe should not care about exception infos, because the old
way of kretprobe is the fixed break-insn-trigger. Then I suggested only
save the CSR.PLV and CSR.PIE. But this is not similar to exception
handler. Each exception occur we should save the current status in case
exception nest. We may unwind and debug the previous exception. If we
hadn't saved status at that time exception occured, we lost them.


Thanks,

Jinyang

>   
> diff --git a/arch/loongarch/kernel/entry.S b/arch/loongarch/kernel/entry.S
> index d53b631..893e632 100644
> --- a/arch/loongarch/kernel/entry.S
> +++ b/arch/loongarch/kernel/entry.S
> @@ -31,14 +31,6 @@ SYM_FUNC_START(handle_syscall)
>   	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