Re: [Xenomai] [PATCH] x86/ipipe: get FPU backup area allocated for all threads

[Jan Kiszka <jan.kiszka@siemens.com>]

On 2013-09-30 21:21, Gilles Chanteperdrix wrote:
> In order to be able to handle FPU faults in the Xenomai domain without
> switching to the root domain, the FPU backup area must be systematically
> allocated for all Linux tasks. Currently, the backup area is systematically
> allocated in arch_dup_task_struct(), but gets freed upon exec by
> flush_thread(). Stop freeing it in flush_thread(), there does not seem
> to be any risk of leaking information, as the "used_math" flag is cleared
> by drop_init_fpui() so that upon next fault, the context is reinitialized.

How to reproduce the scenario? Would be good to document it in order to
check for regressions in future kernels.

> ---
>  arch/x86/kernel/process.c |   10 +---------
>  1 file changed, 1 insertion(+), 9 deletions(-)
> 
> diff --git a/arch/x86/kernel/process.c b/arch/x86/kernel/process.c
> index a00bed1..4716eb2 100644
> --- a/arch/x86/kernel/process.c
> +++ b/arch/x86/kernel/process.c
> @@ -73,14 +73,6 @@ int arch_dup_task_struct(struct task_struct *dst, struct task_struct *src)
>  		if (ret)
>  			return ret;
>  		fpu_copy(dst, src);
> -	} else {
> -#ifdef CONFIG_IPIPE
> -		/* unconditionally allocate, RT domain may need it */
> -		memset(&dst->thread.fpu, 0, sizeof(dst->thread.fpu));
> -		ret = fpu_alloc(&dst->thread.fpu);
> -		if (ret)
> -			return ret;
> -#endif

Why removing the systematic allocation? This is not explained above and
seems counterproductive.

>  	}
>  	return 0;
>  }
> @@ -170,8 +162,8 @@ void flush_thread(void)
>  	 * lazily at the first use.
>  	 */
>  	if (!use_eager_fpu())
> -#endif
>  		free_thread_xstate(tsk);
> +#endif

Yes, this makes sense. It was wrong in multiple ways.

>  }
>  
>  static void hard_disable_TSC(void)
> 

Jan

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