linux-kernel - Re: [PATCH V2] riscv: set trap vector earlier

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Date: Tue, 7 May 2024 07:28:58 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: "yang.zhang" <gaoshanliukou@....com>, alexghiti@...osinc.com
Cc: paul.walmsley@...ive.com, palmer@...belt.com, aou@...s.berkeley.edu,
 linux-riscv@...ts.infradead.org, linux-kernel@...r.kernel.org,
 "yang.zhang" <yang.zhang@...intek.com>
Subject: Re: [PATCH V2] riscv: set trap vector earlier

Hi Yang,

On 06/05/2024 04:22, yang.zhang wrote:
> From: "yang.zhang" <yang.zhang@...intek.com>
>
> If trap earlier, trap vector doesn't yet set properly, current
> value maybe set by previous firmwire, typically it's the _start of kernel,
> it's confused and difficult to debuge, so set it earlier.


What about:

"The exception vector of the booting hart is not set before enabling the 
mmu and then still points to the value of the previous firmware, 
typically _start_kernel. That makes it hard to debug setup_vm() when bad 
things happen. So fix that by setting the exception vector earlier."


> ---
> v1 -> v2:
> As Alex commented, remove the patch for supporting hugesize kernek image
> Add the omissive logic of set trap vector earlier
>
> Signed-off-by: yang.zhang <yang.zhang@...intek.com>
> ---
>   arch/riscv/kernel/head.S | 3 +++
>   1 file changed, 3 insertions(+)
>
> diff --git a/arch/riscv/kernel/head.S b/arch/riscv/kernel/head.S
> index 4236a69c35cb..03dc440e643e 100644
> --- a/arch/riscv/kernel/head.S
> +++ b/arch/riscv/kernel/head.S
> @@ -300,6 +300,9 @@ SYM_CODE_START(_start_kernel)
>   #else
>   	mv a0, a1
>   #endif /* CONFIG_BUILTIN_DTB */
> +	/* Set trap vector to spin forever to help debug */
> +	la a3, .Lsecondary_park
> +	csrw CSR_TVEC, a3
>   	call setup_vm
>   #ifdef CONFIG_MMU
>   	la a0, early_pg_dir

Note that this patch will allow to catch any exception happening in 
setup_vm() but as we reset stvec in relocate_enable_mmu. But that's ok, 
it will help catching weird bugs more easily :)

With the commit message fixed, you can add:

Reviewed-by: Alexandre Ghiti <alexghiti@...osinc.com>

Thanks,

Alex