Re: [RFC PATCH 1/2] x86/ibpb: Skip IBPB when we switch back to same user process

[Peter Zijlstra <peterz@infradead.org>]

On Wed, Jan 24, 2018 at 04:36:41PM -0800, Tim Chen wrote:
> These two patches provide optimization to skip IBPB for this
> commonly encountered scenario:
> We could switch to a kernel idle thread and then back to the original
> process such as:
> process A -> idle -> process A
> 
> In such scenario, we do not have to do IBPB here even though the process
> is non-dumpable, as we are switching back to the same process after
> an hiatus.
> 
> The cost is to have an extra pointer to track the last mm we were using before
> switching to the init_mm used by idle.  But avoiding the extra IBPB
> is probably worth the extra memory for such a common scenario.

So we already track active_mm for kernel threads. I can't immediately
see where this fails for idle and your changelog doesn't say.

> @@ -229,15 +230,17 @@ void switch_mm_irqs_off(struct mm_struct *prev, struct mm_struct *next,
>                   * As an optimization flush indirect branches only when
>                   * switching into processes that disable dumping.
>                   *
> -                 * This will not flush branches when switching into kernel
> -                 * threads, but it would flush them when switching to the
> -                 * idle thread and back.
> +		 * This will not flush branches when switching into kernel
> +		 * threads. It will also not flush if we switch to idle
> +		 * thread and back to the same process. It will flush if we
> +		 * switch to a different non-dumpable process.

Whitespace damage.

>                   *
>                   * It might be useful to have a one-off cache here
>                   * to also not flush the idle case, but we would need some
>                   * kind of stable sequence number to remember the previous mm.
>  		 */
> -		if (tsk && tsk->mm && get_dumpable(tsk->mm) != SUID_DUMP_USER)
> +		if (tsk && tsk->mm && (tsk->mm != last)
> +			&& get_dumpable(tsk->mm) != SUID_DUMP_USER)

Broken coding style, operators go at the end of the previous line.

>  			indirect_branch_prediction_barrier();
>  
>  		if (IS_ENABLED(CONFIG_VMAP_STACK)) {