linux-kernel - Re: [PATCH] riscv: uaccess: Only restore the CSR

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Message-ID: <2c07d65e-0641-42a0-9eb4-9e42d9325ff2@ghiti.fr>
Date: Mon, 26 May 2025 22:39:35 +0200
From: Alexandre Ghiti <alex@...ti.fr>
To: Cyril Bur <cyrilbur@...storrent.com>, samuel.holland@...ive.com,
 ben.dooks@...ethink.co.uk, palmer@...belt.com, linux-kernel@...r.kernel.org
Cc: jszhang@...nel.org, paul.walmsley@...ive.com, charlie@...osinc.com,
 jrtc27@...c27.com, aou@...s.berkeley.edu, linux-riscv@...ts.infradead.org,
 Andy Chiu <andybnac@...il.com>, Deepak Gupta <debug@...osinc.com>
Subject: Re: [PATCH] riscv: uaccess: Only restore the CSR_STATUS SUM bit

+cc linux-riscv, Andy, Deepak

On 5/22/25 18:09, Cyril Bur wrote:
> During switch to csrs will OR the value of the register into the
> corresponding csr. In this case we're only interested in restoring the
> SUM bit not the entire register.
>
> Fixes: 788aa64c0 ("riscv: save the SR_SUM status over switches")
> Signed-off-by: Cyril Bur <cyrilbur@...storrent.com>
> ---
> I've put the Fixes tag in but I assume this will get squashed into the
> patch. Either way I hope this works to fix the immediate issue.
>
>   arch/riscv/kernel/entry.S | 12 ++++++++----
>   1 file changed, 8 insertions(+), 4 deletions(-)
>
> diff --git a/arch/riscv/kernel/entry.S b/arch/riscv/kernel/entry.S
> index 00bd0de9faa2..6ed3bd80903d 100644
> --- a/arch/riscv/kernel/entry.S
> +++ b/arch/riscv/kernel/entry.S
> @@ -399,14 +399,18 @@ SYM_FUNC_START(__switch_to)
>   	REG_S s11, TASK_THREAD_S11_RA(a3)
>   
>   	/* save the user space access flag */
> -	li    s0, SR_SUM
> -	csrr  s1, CSR_STATUS
> -	REG_S s1, TASK_THREAD_STATUS_RA(a3)
> +	csrr  s0, CSR_STATUS
> +	REG_S s0, TASK_THREAD_STATUS_RA(a3)
>   
>   	/* Save the kernel shadow call stack pointer */
>   	scs_save_current
> -	/* Restore context from next->thread */
> +	/*
> +	 * Restore context from next->thread. csrs will OR the bits from s0 and
> +	 * only want to restore the SR_SUM bit
> +	 */
>   	REG_L s0,  TASK_THREAD_STATUS_RA(a4)
> +	li    s1,  SR_SUM
> +	and   s0,  s0, s1
>   	csrs  CSR_STATUS, s0
>   	REG_L ra,  TASK_THREAD_RA_RA(a4)
>   	REG_L sp,  TASK_THREAD_SP_RA(a4)

To conclude the discussion we had here 
https://lore.kernel.org/linux-riscv/aDCtATl2N21fBsyT@debug.ba.rivosinc.com/#t, 
in addition to Cyril's patch above, to me we only have to rename the 
status field into sum and we're good to go. @Andy, @Deepak @Samuel Do 
you agree?

As this is an important fix (along with 2 other fixes, one for thead 
vector and vdso static values), I'd like to send another PR soon for 
inclusion in 6.16-rc1, I did not want to delay the second PR any longer.

Thanks for your feedbacks,

Alex