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)) {
