Re: [PATCH 5/6] powerpc/mm: Optimize detection of thread local mm's

[Frederic Barrat <fbarrat@linux.vnet.ibm.com>]

Le 24/08/2017 à 20:47, Benjamin Herrenschmidt a écrit :
> On Thu, 2017-08-24 at 18:40 +0200, Frederic Barrat wrote:
>>
>> The decrementing part is giving me troubles, and I think it makes sense:
>> if I decrement the counter when detaching the context from the capi
>> card, then the next TLBIs for the memory context may be back to local.
> 
> Yes, you need to flush the CAPI TLB first.
> 
>> So when the process exits, the NPU wouldn't get the associated TLBIs,
>> which spells trouble the next time the same memory context ID is reused.
>> I believe this the cause of the problem I'm seeing. As soon as I keep
>> the TLBIs global, even after I detach from the capi adapter, everything
>> is fine.
>>
>> Does it sound right?
>>
>> So to keep the checks minimal in mm_is_thread_local(), to just checking
>> the active_cpus count, I'm thinking of introducing a "copro enabled" bit
>> on the context, so that we can increment active_cpus only once. And
>> never decrement it.
> 
> You can decrement if you flush. Don't you have MMIOs to do directed
> flushes ?

That's for the nMMU. Last I heard, we don't have MMIOs to flush anything 
on the nMMU.

Side note: for the PSL, we do have MMIOs to flush, but they were 
perceived as useful only for debug and we don't rely on them, precisely 
because the nMMU would fall out of sync, so we have to rely on broadcast.

   Fred