Re: [PATCH V5 13/21] riscv: compat: process: Add UXL_32 support in start_thread

[Palmer Dabbelt]

On Tue, 01 Feb 2022 07:05:37 PST (-0800), guoren@xxxxxxxxxx wrote:
> From: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
>
> If the current task is in COMPAT mode, set SR_UXL_32 in status for
> returning userspace. We need CONFIG _COMPAT to prevent compiling
> errors with rv32 defconfig.
>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxxxxxxxxx>
> Signed-off-by: Guo Ren <guoren@xxxxxxxxxx>
> Cc: Arnd Bergmann <arnd@xxxxxxxx>
> Cc: Palmer Dabbelt <palmer@xxxxxxxxxxx>
> ---
>  arch/riscv/kernel/process.c | 5 +++++
>  1 file changed, 5 insertions(+)
>
> diff --git a/arch/riscv/kernel/process.c b/arch/riscv/kernel/process.c
> index 03ac3aa611f5..1a666ad299b4 100644
> --- a/arch/riscv/kernel/process.c
> +++ b/arch/riscv/kernel/process.c
> @@ -97,6 +97,11 @@ void start_thread(struct pt_regs *regs, unsigned long pc,
>  	}
>  	regs->epc = pc;
>  	regs->sp = sp;
> +
> +#ifdef CONFIG_COMPAT
> +	if (is_compat_task())
> +		regs->status |= SR_UXL_32;

Not sure if I'm just misunderstanding the bit ops here, but aren't we 
trying to set the UXL field to 1 (for UXL=32)?  That should be a bit 
field set op, not just an OR.

> +#endif
>  }
>
>  void flush_thread(void)

Additionally: this isn't really an issue so much with this patch, but it 
does bring up that we're relying on someone else to have set UXL=64 on 
CONFIG_COMPAT=n systems.  I don't see that in any spec anywhere, so we 
should really be setting UXL in Linux for all systems (ie, not just those with
COMPAT=y).