linux-kernel - Re: [PATCH v3 01/29] riscv: envcfg save and restore on task switching

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Date: Wed, 8 May 2024 17:10:46 -0700
From: Charlie Jenkins <charlie@...osinc.com>
To: Deepak Gupta <debug@...osinc.com>
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Subject: Re: [PATCH v3 01/29] riscv: envcfg save and restore on task switching

On Wed, Apr 03, 2024 at 04:34:49PM -0700, Deepak Gupta wrote:
> envcfg CSR defines enabling bits for cache management instructions and
> soon will control enabling for control flow integrity and pointer
> masking features.
> 
> Control flow integrity enabling for forward cfi and backward cfi are
> controlled via envcfg and thus need to be enabled on per thread basis.
> 
> This patch creates a place holder for envcfg CSR in `thread_info` and
> adds logic to save and restore on task switching.
> 
> Signed-off-by: Deepak Gupta <debug@...osinc.com>
> ---
>  arch/riscv/include/asm/switch_to.h   | 10 ++++++++++
>  arch/riscv/include/asm/thread_info.h |  1 +
>  2 files changed, 11 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/switch_to.h b/arch/riscv/include/asm/switch_to.h
> index 7efdb0584d47..2d9a00a30394 100644
> --- a/arch/riscv/include/asm/switch_to.h
> +++ b/arch/riscv/include/asm/switch_to.h
> @@ -69,6 +69,15 @@ static __always_inline bool has_fpu(void) { return false; }
>  #define __switch_to_fpu(__prev, __next) do { } while (0)
>  #endif
>  
> +static inline void __switch_to_envcfg(struct task_struct *next)
> +{
> +	register unsigned long envcfg = next->thread_info.envcfg;

This doesn't need the register storage class.

> +
> +	asm volatile (ALTERNATIVE("nop", "csrw " __stringify(CSR_ENVCFG) ", %0", 0,
> +							  RISCV_ISA_EXT_XLINUXENVCFG, 1)
> +							  :: "r" (envcfg) : "memory");
> +}
> +

Something like:

static inline void __switch_to_envcfg(struct task_struct *next)
{
	if (riscv_has_extension_unlikely(RISCV_ISA_EXT_XLINUXENVCFG))
		csr_write(CSR_ENVCFG, next->thread_info.envcfg);
}

would be easier to read, but the alternative you have written doesn't
have the jump that riscv_has_extension_unlikely has so what you have
will be more performant.

Does envcfg need to be save/restored always or just with
CONFIG_RISCV_USER_CFI?

- Charlie

>  extern struct task_struct *__switch_to(struct task_struct *,
>  				       struct task_struct *);
>  
> @@ -80,6 +89,7 @@ do {							\
>  		__switch_to_fpu(__prev, __next);	\
>  	if (has_vector())					\
>  		__switch_to_vector(__prev, __next);	\
> +	__switch_to_envcfg(__next);				\
>  	((last) = __switch_to(__prev, __next));		\
>  } while (0)
>  
> diff --git a/arch/riscv/include/asm/thread_info.h b/arch/riscv/include/asm/thread_info.h
> index 5d473343634b..a503bdc2f6dd 100644
> --- a/arch/riscv/include/asm/thread_info.h
> +++ b/arch/riscv/include/asm/thread_info.h
> @@ -56,6 +56,7 @@ struct thread_info {
>  	long			user_sp;	/* User stack pointer */
>  	int			cpu;
>  	unsigned long		syscall_work;	/* SYSCALL_WORK_ flags */
> +	unsigned long envcfg;
>  #ifdef CONFIG_SHADOW_CALL_STACK
>  	void			*scs_base;
>  	void			*scs_sp;
> -- 
> 2.43.2
>