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

[Nadav Amit]

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".
> 
> Right?
This approach may work, but I doubt other_cpus_need_tlb_flush() would
be needed by anyone, excluding this "hacky" workaround. There are already
five interfaces for invalidation of: a single page, a userspace range,
a whole task, a kernel range, and full flush including kernel (global)
entries.

> 
>> In theory, fix_pte_leak could have used flush_tlb_page. But the problem
>> is that flush_tlb_page requires the vm_area_struct as an argument, which
>> ptep_get_and_clear (and others) do not have.
> 
> That, and we do not want/need to flush the _current_ processor's TLB.
> flush_tlb_page() would have done that unnecessarily.  That's not the end
> of the world here, but it is a downside.

Oops, I missed the fact a local flush is not needed in this case.

Nadav