Re: [PATCH] Linux VM workaround for Knights Landing A/D leak

[Andy Lutomirski]

On Tue, Jun 14, 2016 at 7:44 PM, Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
> Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>
>> On Tue, Jun 14, 2016 at 7:35 PM, Nadav Amit <nadav.amit@xxxxxxxxx> wrote:
>>> Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
>>>
>>>> On Tue, Jun 14, 2016 at 2:37 PM, Dave Hansen
>>>> <dave.hansen@xxxxxxxxxxxxxxx> wrote:
>>>>> On 06/14/2016 01:16 PM, Nadav Amit wrote:
>>>>>> Dave Hansen <dave.hansen@xxxxxxxxxxxxxxx> wrote:
>>>>>>
>>>>>>> On 06/14/2016 09:47 AM, Nadav Amit wrote:
>>>>>>>> Lukasz Anaczkowski <lukasz.anaczkowski@xxxxxxxxx> wrote:
>>>>>>>>
>>>>>>>>>> From: Andi Kleen <ak@xxxxxxxxxxxxxxx>
>>>>>>>>>> +void fix_pte_leak(struct mm_struct *mm, unsigned long addr, pte_t *ptep)
>>>>>>>>>> +{
>>>>>>>> Here there should be a call to smp_mb__after_atomic() to synchronize with
>>>>>>>> switch_mm. I submitted a similar patch, which is still pending (hint).
>>>>>>>>
>>>>>>>>>> + if (cpumask_any_but(mm_cpumask(mm), smp_processor_id()) < nr_cpu_ids) {
>>>>>>>>>> +         trace_tlb_flush(TLB_LOCAL_SHOOTDOWN, TLB_FLUSH_ALL);
>>>>>>>>>> +         flush_tlb_others(mm_cpumask(mm), mm, addr,
>>>>>>>>>> +                          addr + PAGE_SIZE);
>>>>>>>>>> +         mb();
>>>>>>>>>> +         set_pte(ptep, __pte(0));
>>>>>>>>>> + }
>>>>>>>>>> +}
>>>>>>>
>>>>>>> Shouldn't that barrier be incorporated in the TLB flush code itself and
>>>>>>> not every single caller (like this code is)?
>>>>>>>
>>>>>>> It is insane to require individual TLB flushers to be concerned with the
>>>>>>> barriers.
>>>>>>
>>>>>> IMHO it is best to use existing flushing interfaces instead of creating
>>>>>> new ones.
>>>>>
>>>>> Yeah, or make these things a _little_ harder to get wrong.  That little
>>>>> snippet above isn't so crazy that we should be depending on open-coded
>>>>> barriers to get it right.
>>>>>
>>>>> Should we just add a barrier to mm_cpumask() itself?  That should stop
>>>>> the race.  Or maybe we need a new primitive like:
>>>>>
>>>>> /*
>>>>> * Call this if a full barrier has been executed since the last
>>>>> * pagetable modification operation.
>>>>> */
>>>>> static int __other_cpus_need_tlb_flush(struct mm_struct *mm)
>>>>> {
>>>>>       /* cpumask_any_but() returns >= nr_cpu_ids if no cpus set. */
>>>>>       return cpumask_any_but(mm_cpumask(mm), smp_processor_id()) <
>>>>>               nr_cpu_ids;
>>>>> }
>>>>>
>>>>>
>>>>> static int other_cpus_need_tlb_flush(struct mm_struct *mm)
>>>>> {
>>>>>       /*
>>>>>        * Synchronizes with switch_mm.  Makes sure that we do not
>>>>>        * observe a bit having been cleared in mm_cpumask() before
>>>>>        * the other processor has seen our pagetable update.  See
>>>>>        * switch_mm().
>>>>>        */
>>>>>       smp_mb__after_atomic();
>>>>>
>>>>>       return __other_cpus_need_tlb_flush(mm)
>>>>> }
>>>>>
>>>>> We should be able to deploy other_cpus_need_tlb_flush() in most of the
>>>>> cases where we are doing "cpumask_any_but(mm_cpumask(mm),
>>>>> smp_processor_id()) < nr_cpu_ids".
>>>>
>>>> IMO this is a bit nuts.  smp_mb__after_atomic() doesn't do anything on
>>>> x86.  And, even if it did, why should the flush code assume that the
>>>> previous store was atomic?
>>>>
>>>> What's the issue being fixed / worked around here?
>>>
>>> It does a compiler barrier, which prevents the decision whether a
>>> remote TLB shootdown is required to be made before the PTE is set.
>>>
>>> I agree that PTEs may not be written atomically in certain cases
>>> (although I am unaware of such cases, except on full-mm flush).
>>
>> How about plain set_pte?  It's atomic (aligned word-sized write), but
>> it's not atomic in the _after_atomic sense.
>
> Can you point me to a place where set_pte is used before a TLB
> invalidation/shootdown, excluding this patch and the fullmm case?
>
> I am not claiming there is no such case, but I am unaware of such
> one. PTEs are cleared on SMP using xchg, and similarly the dirty bit
> is cleared with an atomic operation.
>

Hmm, you may be right.  I still think this is all disgusting, but I
don't have any better ideas.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC