linux-kernel - Re: [PATCH v9 07/17] riscv: Reset vector register

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Date:   Tue, 14 Dec 2021 08:29:28 -0800 (PST)
From:   Palmer Dabbelt <palmer@...belt.com>
To:     greentime.hu@...ive.com
CC:     Paul Walmsley <paul.walmsley@...ive.com>,
        linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
        aou@...s.berkeley.edu
Subject:     Re: [PATCH v9 07/17] riscv: Reset vector register

On Tue, 09 Nov 2021 01:48:19 PST (-0800), greentime.hu@...ive.com wrote:
> From: Guo Ren <guoren@...ux.alibaba.com>
>
> Reset vector registers at boot-time and disable vector instructions
> execution for kernel mode.
>
> Signed-off-by: Guo Ren <guoren@...ux.alibaba.com>
> Co-developed-by: Vincent Chen <vincent.chen@...ive.com>
> Signed-off-by: Vincent Chen <vincent.chen@...ive.com>
> Co-developed-by: Han-Kuan Chen <hankuan.chen@...ive.com>
> Signed-off-by: Han-Kuan Chen <hankuan.chen@...ive.com>
> Co-developed-by: Greentime Hu <greentime.hu@...ive.com>
> Signed-off-by: Greentime Hu <greentime.hu@...ive.com>
> ---
>  arch/riscv/kernel/entry.S |  6 +++---
>  arch/riscv/kernel/head.S  | 22 ++++++++++++++++++++--
>  2 files changed, 23 insertions(+), 5 deletions(-)
>
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 98f502654edd..ad0fa80ada81 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -77,10 +77,10 @@ _save_context:
>  	 * Disable user-mode memory access as it should only be set in the
>  	 * actual user copy routines.
>  	 *
> -	 * Disable the FPU to detect illegal usage of floating point in kernel
> -	 * space.
> +	 * Disable the FPU/Vector to detect illegal usage of floating point
> +	 * or vector in kernel space.
>  	 */
> -	li t0, SR_SUM | SR_FS
> +	li t0, SR_SUM | SR_FS | SR_VS
>
>  	REG_L s0, TASK_TI_USER_SP(tp)
>  	csrrc s1, CSR_STATUS, t0
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 52c5ff9804c5..551afe1de85e 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -242,10 +242,10 @@ pmp_done:
>  .option pop
>
>  	/*
> -	 * Disable FPU to detect illegal usage of
> +	 * Disable FPU & VECTOR to detect illegal usage of
>  	 * floating point in kernel space

Presumably that should be "floating point or vector", like the other 
one?

>  	 */
> -	li t0, SR_FS
> +	li t0, SR_FS | SR_VS
>  	csrc CSR_STATUS, t0
>
>  #ifdef CONFIG_SMP
> @@ -433,6 +433,24 @@ ENTRY(reset_regs)
>  	csrw	fcsr, 0
>  	/* note that the caller must clear SR_FS */
>  #endif /* CONFIG_FPU */
> +
> +#ifdef CONFIG_VECTOR
> +	csrr	t0, CSR_MISA
> +	li	t1, (COMPAT_HWCAP_ISA_V >> 16)
> +	slli	t1, t1, 16

Why?  Shouldn't the "li" pseudo handle generating that constant fine?  
It generates the expected lui for me.

> +	and	t0, t0, t1
> +	beqz	t0, .Lreset_regs_done
> +
> +	li	t1, SR_VS
> +	csrs	CSR_STATUS, t1
> +	vsetvli t1, x0, e8, m8
> +	vmv.v.i v0, 0
> +	vmv.v.i v8, 0
> +	vmv.v.i v16, 0
> +	vmv.v.i v24, 0

I don't see anything resetting vcsr here, which is explicitly required 
by ISA manual.

Otherwise this looks OK to me: I wasn't actually sure this was guaranteed 
to hit every bit in the vector register file, but IIUC it does -- VLMAX 
has a defined value, VLEN is a constant, and this form of vsetvli is 
defined to set vl to VLMAX.  Probably worth a comment, though.

> +	/* note that the caller must clear SR_VS */
> +#endif /* CONFIG_VECTOR */
> +
>  .Lreset_regs_done:
>  	ret
>  END(reset_regs)

With those minor bits fixed,

Reviewed-by: Palmer Dabbelt <palmer@...osinc.com>