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

[Tim Chen]

On 01/25/2018 10:18 AM, Peter Zijlstra wrote:
> On Thu, Jan 25, 2018 at 09:04:21AM -0800, Andy Lutomirski wrote:
>> I haven't tried to fully decipher the patch, but I think the idea is
>> wrong.  (I think it's the same wrong idea that Rik and I both had and
>> that I got into Linus' tree for a while...)  The problem is that it's
>> not actually correct to run indefinitely in kernel mode using stale
>> cached page table data.  The stale PTEs themselves are fine, but the
>> stale intermediate translations can cause the CPU to speculatively
>> load complete garbage into the TLB, and that's bad (and causes MCEs on
>> AMD CPUs).
> 
> Urggh.. indeed :/
> 
>> I think we only really have two choices: tlb_defer_switch_to_init_mm()
>> == true and tlb_defer_switch_to_init_mm() == false.  The current
>> heuristic is to not defer if we have PCID, because loading CR3 is
>> reasonably fast.
> 
> I just _really_ _really_ hate idle drivers doing leave_mm(). I don't
> suppose limiting the !IPI case to just the idle case would be correct
> either, because between waking from idle and testing our 'should I have
> invalidated' bit it can (however unlikely) speculate into stale TLB
> entries too..
> 
> 

Peter, 

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?

Tim