linux-kernel - Re: [PATCH v12 06/17] riscv: Reset vector register

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Date:   Thu, 3 Nov 2022 22:01:27 -0700
From:   Vineet Gupta <vineet.gupta@...ux.dev>
To:     Chris Stillson <stillson@...osinc.com>
Cc:     Guo Ren <guoren@...ux.alibaba.com>,
        Vincent Chen <vincent.chen@...ive.com>,
        Han-Kuan Chen <hankuan.chen@...ive.com>,
        Greentime Hu <greentime.hu@...ive.com>,
        Palmer Dabbelt <palmer@...osinc.com>,
        Paul Walmsley <paul.walmsley@...ive.com>,
        Palmer Dabbelt <palmer@...belt.com>,
        Albert Ou <aou@...s.berkeley.edu>,
        Eric Biederman <ebiederm@...ssion.com>,
        Kees Cook <keescook@...omium.org>,
        Anup Patel <anup@...infault.org>,
        Atish Patra <atishp@...shpatra.org>,
        Oleg Nesterov <oleg@...hat.com>,
        Heinrich Schuchardt <heinrich.schuchardt@...onical.com>,
        Guo Ren <guoren@...nel.org>,
        Mayuresh Chitale <mchitale@...tanamicro.com>,
        Paolo Bonzini <pbonzini@...hat.com>,
        linux-riscv <linux-riscv@...ts.infradead.org>,
        lkml <linux-kernel@...r.kernel.org>,
        Andy Chiu <andy.chiu@...ive.com>
Subject: Re: [PATCH v12 06/17] riscv: Reset vector register

On 9/21/22 14:43, Chris Stillson wrote:
> From: Guo Ren <guoren@...ux.alibaba.com>
> 
> Reset vector registers at boot-time and disable vector instructions
> execution for kernel mode.

Perhaps bike-shedding, but "Reset" has a different connotation in 
kernel, this is clear registers IMO. And "Reset Vector ..." sounds 
totally different at first glance.


> -	 * 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

Is VS writable in implementations not implementing V hardware.

Priv spec seems to be confusing. It states

    "The FS[1:0] and VS[1:0] WARL fields..."

Above implies it can be written always but will read legal values only.
But then this follows.

	"If neither the v registers nor S-mode is implemented, then VS 	
	is read-only zero. If S-mode is implemented but the v registers
	are not, VS may optionally be read-only zero"

What does optionally mean for software ?

>   
>   	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 b865046e4dbb..2c81ca42ec4e 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -140,10 +140,10 @@ secondary_start_sbi:
>   	.option pop
>   
>   	/*
> -	 * Disable FPU to detect illegal usage of
> -	 * floating point in kernel space
> +	 * Disable FPU & VECTOR to detect illegal usage of
> +	 * floating point or vector in kernel space
>   	 */
> -	li t0, SR_FS
> +	li t0, SR_FS | SR_VS
>   	csrc CSR_STATUS, t0
>   
>   	/* Set trap vector to spin forever to help debug */
> @@ -234,10 +234,10 @@ pmp_done:
>   .option pop
>   
>   	/*
> -	 * Disable FPU to detect illegal usage of
> -	 * floating point in kernel space
> +	 * Disable FPU & VECTOR to detect illegal usage of
> +	 * floating point or vector in kernel space
>   	 */
> -	li t0, SR_FS
> +	li t0, SR_FS | SR_VS
>   	csrc CSR_STATUS, t0

Third instance of duplicated SR_FS | SR_VS. Better to add a helper 
SR_FS_VS or some such macro.

>   
>   #ifdef CONFIG_RISCV_BOOT_SPINWAIT
> @@ -431,6 +431,29 @@ 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
> +	and	t0, t0, t1
> +	beqz	t0, .Lreset_regs_done
> +
> +	/*
> +	 * Clear vector registers and reset vcsr
> +	 * VLMAX has a defined value, VLEN is a constant,
> +	 * and this form of vsetvli is defined to set vl to VLMAX.
> +	 */
> +	li	t1, SR_VS
> +	csrs	CSR_STATUS, t1
> +	csrs	CSR_VCSR, x0
> +	vsetvli t1, x0, e8, m8, ta, ma
> +	vmv.v.i v0, 0
> +	vmv.v.i v8, 0
> +	vmv.v.i v16, 0
> +	vmv.v.i v24, 0
> +	/* note that the caller must clear SR_VS */

Is that actually happening ?