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

[Dave Hansen]

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?

> 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.