On Thu, Jan 25, 2018 at 11:32:46AM -0800, Tim Chen wrote:
>
> This patch is not ideal as it comes with the caveats that
> patch 2 tries to close. I put it out here to see if it can prompt
> people to come up with a better solution. Keeping active_mm around would
> have been cleaner but it looks like there are issues that Andy mentioned.
>
> The "A -> idle -> A" case would not trigger IBPB if
> tlb_defer_switch_to_init_mm()
> is true (non pcid) as we does not change the mm.
>
> This patch tries to address the case when we do switch to init_mm and back.
> Do you still have objections to the approach in this patch
> to save the last active mm before switching to init_mm?
I still think the existing active_mm is sufficient. Something like:
switch_mm()
{
...
if (prev && next != prev)
ibpb();
...
}
should work. Because while the idle crud does leave_mm() and PCID does
enter_lazy_tlb() and both end up doing: switch_mm(NULL, &init_mm, NULL),
nothing there affects tsk->active_mm.
So over the "A -> idle -> A" transition, active_mm should actually track
what you want.
