Re: [PATCH] drm/ttm: pass buffer object for bind/unbind callback

[Thomas Hellstrom <thellstrom@vmware.com>]

On 11/19/2011 01:26 AM, Ben Skeggs wrote:
> On Fri, 2011-11-18 at 23:48 +0100, Thomas Hellstrom wrote:
>    
>> On 11/18/2011 06:26 PM, Ben Skeggs wrote:
>>      
>>> On Fri, 2011-11-18 at 15:30 +0100, Thomas Hellstrom wrote:
>>>
>>>        
>>>> On 11/18/2011 02:15 PM, Ben Skeggs wrote:
>>>>
>>>>          
>>>>> On Fri, 2011-11-18 at 08:57 +0100, Thomas Hellstrom wrote:
>>>>>
>>>>>
>>>>>            
>>>>>> Jerome,
>>>>>>
>>>>>> I don't like this change for the following reasons
>>>>>>
>>>>>>
>>>>>>              
>>>>> -snip-
>>>>>
>>>>>
>>>>>
>>>>>            
>>>>>>> One can take advantage of move notify callback but, there are
>>>>>>> corner case where bind/unbind might be call without move notify
>>>>>>> being call (in error path mostly). So to make sure that each
>>>>>>> virtual address space have a consistent view of wether a buffer
>>>>>>> object is backed or not by system page it's better to pass the
>>>>>>> buffer object to bind/unbind.
>>>>>>>
>>>>>>>
>>>>>>>                
>>>>> As I discussed previously with Jerome on IRC, I believe the move_notify
>>>>> hook is sufficient.  I fixed a couple of issues back with it back when I
>>>>> implemented support for this in nouveau.
>>>>>
>>>>> Jerome did point out two issues however, which I believe can be solved
>>>>> easily enough.
>>>>>
>>>>> The first was that the error path after move_notify() has been called
>>>>> doesn't reverse the move_notify() too, leaving incorrect page table
>>>>> entries.  This, I think, could easily be solved by swapping bo->mem and
>>>>> mem, and re-calling move_notify() to have the driver reverse whatever it
>>>>> did.
>>>>>
>>>>> The second issue is that apparently move_notify() doesn't get called in
>>>>> the destroy path.  Nouveau's GEM layer takes care of this for our
>>>>> userspace bos, and we don't use any kernel bos that are mapped into a
>>>>> channel's address space so we don't hit it.  This can probably be solved
>>>>> easily enough too I expect.
>>>>>
>>>>> Ben.
>>>>>
>>>>>
>>>>>
>>>>>            
>>>> OK. I understand. Surely if a move_notify is missing somewhere, that can
>>>> easily be fixed.
>>>>
>>>> It might be good if we can outline how the virtual tables are set up. In
>>>> my world, the following would happen:
>>>>
>>>> 1) Pre command submission:
>>>>
>>>> a) reserve bo
>>>> b) call ttm_bo_validate to set placement. move_notify will tear down any
>>>> existing GPU page tables for the bo.
>>>> c) Set up GPU page tables.
>>>>
>>>>          
>>> Nouveau doesn't do this exactly.  I wanted to avoid having to check if a
>>> bo was bound to a particular address space on every command submission.
>>>
>>>        
>> It perhaps might be a good idea to revisit this?
>> I think using move_notify to set up a new placement before the data has
>> actually been moved is very fragile and not the intended use. That would
>> also save you from having to handle error paths. Also how do you handle
>> swapouts?
>>      
> I don't see how it's fragile, there's only the one error path after that
> point that needs to undo it.  And, what *is* the expected use then?  I
> see it as "tell the driver the buffer is moving so it can do whatever is
> necessary as a result".
>
> Swapouts are a missed case though, indeed.
>
>    
>> A quick check in c) that the virtual map hasn't been torn down by a
>> move_notify and, in that case, rebuild it would probably be next to
>> free. The virtual GPU mapping would then be valid only after validation
>> and while the bo is fenced or pinned.
>>      
> This alone doesn't solve the swapouts issue either unless you're also
> wanting us to unmap once a buffer becomes unfenced, which would lead to
> loads of completely unnecessary map/unmaps.
>
> Ben.
>    

I think you misunderstand me a little.
The swapout issue should be handled by calling a move_notify() to kill 
the virtual GPU mappings.

However, when moving data that has page tables pointing to it, one 
should (IMHO) do:

1) Kill the old page tables
2) Move the object
3) Set up new page tables on demand.

This is done by TTM for CPU page tables and I think that should be done 
also for GPU page tables. move_notify() should be responsible for 1), 
The driver execbuf for 3), and 3) needs only to be done for the 
particular ring / fifo which is executing commands that touch the buffer.

This leaves a clear and well-defined requirement on TTM:
Notify the driver when it needs to kill its GPU page tables.

With the latest patch from jerome:
Notify the driver when it needs to rebuild it page tables. Also on error 
paths, but not for swapins because no driver will probably set up GPU 
page tables to SYSTEM_MEMORY.

This is what I mean with fragile, and I much rather see the other approach.

Ben, I didn't fully understand why you didn't want to use that approach. 
Did you see a significant overhead with it.

Thanks,
/Thomas