[Mesa-dev] Linux Graphics Next: Userspace submission update

[Christian König]

Am 09.06.21 um 15:19 schrieb Daniel Vetter:
> [SNIP]
>> Yeah, we call this the lightweight and the heavyweight tlb flush.
>>
>> The lighweight can be used when you are sure that you don't have any of the
>> PTEs currently in flight in the 3D/DMA engine and you just need to
>> invalidate the TLB.
>>
>> The heavyweight must be used when you need to invalidate the TLB *AND* make
>> sure that no concurrently operation moves new stuff into the TLB.
>>
>> The problem is for this use case we have to use the heavyweight one.
> Just for my own curiosity: So the lightweight flush is only for in-between
> CS when you know access is idle? Or does that also not work if userspace
> has a CS on a dma engine going at the same time because the tlb aren't
> isolated enough between engines?

More or less correct, yes.

The problem is a lightweight flush only invalidates the TLB, but doesn't 
take care of entries which have been handed out to the different engines.

In other words what can happen is the following:

1. Shader asks TLB to resolve address X.
2. TLB looks into its cache and can't find address X so it asks the 
walker to resolve.
3. Walker comes back with result for address X and TLB puts that into 
its cache and gives it to Shader.
4. Shader starts doing some operation using result for address X.
5. You send lightweight TLB invalidate and TLB throws away cached values 
for address X.
6. Shader happily still uses whatever the TLB gave to it in step 3 to 
accesses address X

See it like the shader has their own 1 entry L0 TLB cache which is not 
affected by the lightweight flush.

The heavyweight flush on the other hand sends out a broadcast signal to 
everybody and only comes back when we are sure that an address is not in 
use any more.

Christian.

> -Daniel
>