Re: [RFC PATCH v2 1/2] x86/speculation: Allow per-process control of when to issue IBPB

[Thomas Gleixner]

Anand,

On Thu, Apr 29 2021 at 18:44, Anand K. Mistry wrote:
>  
> -static inline unsigned long mm_mangle_tif_spec_ib(struct task_struct *next)
> +static inline unsigned long mm_mangle_tif_spec_ib_on_leave(
> +	struct task_struct *next)
>  {
>  	unsigned long next_tif = task_thread_info(next)->flags;
> -	unsigned long ibpb = (next_tif >> TIF_SPEC_IB) & LAST_USER_MM_IBPB;
> +	unsigned long spec_disabled =
> +		(next_tif >> TIF_SPEC_IB) & ~(next_tif >> TIF_NO_IBPB_LEAVE);
> +	unsigned long ibpb = spec_disabled & LAST_USER_MM_IBPB;
>  
>  	return (unsigned long)next->mm | ibpb;
>  }
>  
> +static inline bool ibpb_on_entry(struct task_struct *next)
> +{
> +	unsigned long next_tif = task_thread_info(next)->flags;
> +	unsigned long spec_disabled =
> +		(next_tif >> TIF_SPEC_IB) & ~(next_tif >> TIF_NO_IBPB_ENTRY);
> +	return spec_disabled & 1;
> +}

Why exactly do we need _three_ TIF bits and this non-intuitive inverted
logic?

The existing mode is: Issue IBPB when scheduling in and when scheduling out.

Ergo the obvious approach for making it more finegrained is to split the
existing TIF_SPEC_IB into TIF_SPEC_IB_IN and TIF_SPEC_IB_OUT and have
the existing mode both bits set.

Thanks,

        tglx