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

[Peter Zijlstra <peterz@infradead.org>]

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.