Re: [PATCH v3] riscv: Discard vector state on syscalls

[Conor Dooley <conor.dooley@microchip.com>]

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Hey,

On Thu, Jun 29, 2023 at 08:27:30AM +0200, Björn Töpel wrote:
> From: Björn Töpel <bjorn@rivosinc.com>
> 
> The RISC-V vector specification states:
>   Executing a system call causes all caller-saved vector registers
>   (v0-v31, vl, vtype) and vstart to become unspecified.
> 
> The vector registers are set to all 1s, vill is set (invalid), and the
> vector status is set to Dirty.
> 
> That way we can prevent userspace from accidentally relying on the
> stated save.
> 
> Rémi pointed out [1] that writing to the registers might be
> superfluous, and setting vill is sufficient.
> 
> Link: https://lore.kernel.org/linux-riscv/12784326.9UPPK3MAeB@basile.remlab.net/ # [1]
> Suggested-by: Darius Rad <darius@bluespec.com>
> Suggested-by: Palmer Dabbelt <palmer@rivosinc.com>
> Suggested-by: Rémi Denis-Courmont <remi@remlab.net>
> Signed-off-by: Björn Töpel <bjorn@rivosinc.com>

clang allmodconfig and rv32_defconfig fail to build with this patch,
according to patchwork:
../arch/riscv/kernel/traps.c:299:3: error: call to undeclared function 'riscv_v_vstate_discard'; ISO C99 and later do not support implicit function declarations [-Wimplicit-function-declaration]

Cheers,
Conor.

> ---
> 
> v2->v3:
>   Set state to Dirty after discard, for proper ptrace() handling
>   (Andy)
> 
> v1->v2:
>   Proper register restore for initial state (Andy)
>   Set registers to 1s, and not 0s (Darius)
> 
> ---
>  arch/riscv/include/asm/vector.h | 33 +++++++++++++++++++++++++++++++++
>  arch/riscv/kernel/traps.c       |  2 ++
>  2 files changed, 35 insertions(+)
> 
> diff --git a/arch/riscv/include/asm/vector.h b/arch/riscv/include/asm/vector.h
> index 04c0b07bf6cd..0b23056503c5 100644
> --- a/arch/riscv/include/asm/vector.h
> +++ b/arch/riscv/include/asm/vector.h
> @@ -33,6 +33,11 @@ static inline void __riscv_v_vstate_clean(struct pt_regs *regs)
>  	regs->status = (regs->status & ~SR_VS) | SR_VS_CLEAN;
>  }
>  
> +static inline void __riscv_v_vstate_dirty(struct pt_regs *regs)
> +{
> +	regs->status = (regs->status & ~SR_VS) | SR_VS_DIRTY;
> +}
> +
>  static inline void riscv_v_vstate_off(struct pt_regs *regs)
>  {
>  	regs->status = (regs->status & ~SR_VS) | SR_VS_OFF;
> @@ -128,6 +133,34 @@ static inline void __riscv_v_vstate_restore(struct __riscv_v_ext_state *restore_
>  	riscv_v_disable();
>  }
>  
> +static inline void __riscv_v_vstate_discard(void)
> +{
> +	unsigned long vl, vtype_inval = 1UL << (BITS_PER_LONG - 1);
> +
> +	riscv_v_enable();
> +	asm volatile (
> +		".option push\n\t"
> +		".option arch, +v\n\t"
> +		"vsetvli	%0, x0, e8, m8, ta, ma\n\t"
> +		"vmv.v.i	v0, -1\n\t"
> +		"vmv.v.i	v8, -1\n\t"
> +		"vmv.v.i	v16, -1\n\t"
> +		"vmv.v.i	v24, -1\n\t"
> +		"vsetvl		%0, x0, %1\n\t"
> +		".option pop\n\t"
> +		: "=&r" (vl) : "r" (vtype_inval) : "memory");
> +	riscv_v_disable();
> +}
> +
> +static inline void riscv_v_vstate_discard(struct pt_regs *regs)
> +{
> +	if ((regs->status & SR_VS) == SR_VS_OFF)
> +		return;
> +
> +	__riscv_v_vstate_discard();
> +	__riscv_v_vstate_dirty(regs);
> +}
> +
>  static inline void riscv_v_vstate_save(struct task_struct *task,
>  				       struct pt_regs *regs)
>  {
> diff --git a/arch/riscv/kernel/traps.c b/arch/riscv/kernel/traps.c
> index 5158961ea977..5ff63a784a6d 100644
> --- a/arch/riscv/kernel/traps.c
> +++ b/arch/riscv/kernel/traps.c
> @@ -296,6 +296,8 @@ asmlinkage __visible __trap_section void do_trap_ecall_u(struct pt_regs *regs)
>  		regs->epc += 4;
>  		regs->orig_a0 = regs->a0;
>  
> +		riscv_v_vstate_discard(regs);
> +
>  		syscall = syscall_enter_from_user_mode(regs, syscall);
>  
>  		if (syscall < NR_syscalls)
> 
> base-commit: 488833ccdcac118da16701f4ee0673b20ba47fe3
> -- 
> 2.39.2
> 

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