Re: [PATCH drm-next v5 03/14] drm: manager to keep track of GPUs VA mappings

[Danilo Krummrich <dakr@redhat.com>]

On 6/22/23 16:42, Christian König wrote:
> Am 22.06.23 um 16:22 schrieb Danilo Krummrich:
>> On 6/22/23 15:54, Christian König wrote:
>>> Am 20.06.23 um 14:23 schrieb Danilo Krummrich:
>>>> Hi Christian,
>>>>
>>>> On 6/20/23 08:45, Christian König wrote:
>>>>> Hi Danilo,
>>>>>
>>>>> sorry for the delayed reply. I've trying to dig myself out of a 
>>>>> hole at the moment.
>>>>
>>>> No worries, thank you for taking a look anyway!
>>>>
>>>>>
>>>>> Am 20.06.23 um 02:42 schrieb Danilo Krummrich:
>>>>>> [SNIP]
>>>>>> diff --git a/include/drm/drm_gem.h b/include/drm/drm_gem.h
>>>>>> index bbc721870c13..5ec8148a30ee 100644
>>>>>> --- a/include/drm/drm_gem.h
>>>>>> +++ b/include/drm/drm_gem.h
>>>>>> @@ -36,6 +36,8 @@
>>>>>>   #include <linux/kref.h>
>>>>>>   #include <linux/dma-resv.h>
>>>>>> +#include <linux/list.h>
>>>>>> +#include <linux/mutex.h>
>>>>>>   #include <drm/drm_vma_manager.h>
>>>>>> @@ -379,6 +381,18 @@ struct drm_gem_object {
>>>>>>        */
>>>>>>       struct dma_resv _resv;
>>>>>> +    /**
>>>>>> +     * @gpuva:
>>>>>> +     *
>>>>>> +     * Provides the list of GPU VAs attached to this GEM object.
>>>>>> +     *
>>>>>> +     * Drivers should lock list accesses with the GEMs &dma_resv 
>>>>>> lock
>>>>>> +     * (&drm_gem_object.resv).
>>>>>> +     */
>>>>>> +    struct {
>>>>>> +        struct list_head list;
>>>>>> +    } gpuva;
>>>>>> +
>>>>>>       /**
>>>>>>        * @funcs:
>>>>>>        *
>>>>>
>>>>> I'm pretty sure that it's not a good idea to attach this directly 
>>>>> to the GEM object.
>>>>
>>>> Why do you think so? IMHO having a common way to connect mappings to 
>>>> their backing buffers is a good thing, since every driver needs this 
>>>> connection anyway.
>>>>
>>>> E.g. when a BO gets evicted, drivers can just iterate the list of 
>>>> mappings and, as the circumstances require, invalidate the 
>>>> corresponding mappings or to unmap all existing mappings of a given 
>>>> buffer.
>>>>
>>>> What would be the advantage to let every driver implement a driver 
>>>> specific way of keeping this connection?
>>>
>>> Flexibility. For example on amdgpu the mappings of a BO are groups by 
>>> VM address spaces.
>>>
>>> E.g. the BO points to multiple bo_vm structures which in turn have 
>>> lists of their mappings.
>>
>> Isn't this (almost) the same relationship I introduce with the GPUVA 
>> manager?
>>
>> If you would switch over to the GPUVA manager right now, it would be 
>> that every GEM has a list of it's mappings (the gpuva list). The 
>> mapping is represented by struct drm_gpuva (of course embedded in 
>> driver specific structure(s)) which has a pointer to the VM address 
>> space it is part of, namely the GPUVA manager instance. And the GPUVA 
>> manager keeps a maple tree of it's mappings as well.
>>
>> If you still would like to *directly* (indirectly you already have 
>> that relationship) keep a list of GPUVA managers (VM address spaces) 
>> per GEM, you could still do that in a driver specific way.
>>
>> Do I miss something?
> 
> How do you efficiently find only the mappings of a BO in one VM?

Actually, I think this case should even be more efficient than with a BO 
having a list of GPUVAs (or mappings):

Having a list of GPUVAs per GEM, each GPUVA has a pointer to it's VM. 
Hence, you'd only need to iterate the list of mappings for a given BO 
and check the mappings VM pointer.

Having a list of VMs per BO, you'd have to iterate the whole VM to find 
the mappings having a pointer to the given BO, right?

I'd think that a single VM potentially has more mapping entries than a 
single BO was mapped in multiple VMs.

Another case to consider is the case I originally had in mind choosing 
this relationship: finding all mappings for a given BO, which I guess 
all drivers need to do in order to invalidate mappings on BO eviction.

Having a list of VMs per BO, wouldn't you need to iterate all of the VMs 
entirely?

> 
> Keep in mind that we have cases where one BO is shared with hundreds of 
> different VMs as well as potentially the number of mappings can be >10k.
> 
>>
>>>
>>> Additional to that there is a state maschine associated with the 
>>> mappings, e.g. if the page tables are up to date or need to be 
>>> updated etc....
>>>
>>>> Do you see cases where this kind of connection between mappings and 
>>>> backing buffers wouldn't be good enough? If so, which cases do you 
>>>> have in mind? Maybe we can cover them in a common way as well?
>>>
>>> Yeah, we have tons of cases like that. But I have no idea how to 
>>> generalize them.
>>
>> They could still remain to be driver specific then, right?
> 
> Well does the mapping has a back pointer to the BO? And can that be 
> optional NULL if there is no BO?

Yes to both.

- Danilo

> 
> Regards,
> Christian.
> 
>>
>>>
>>>>
>>>>>
>>>>> As you wrote in the commit message it's highly driver specific what 
>>>>> to map and where to map it.
>>>>
>>>> In the end the common case should be that in a VA space at least 
>>>> every mapping being backed by a BO is represented by a struct 
>>>> drm_gpuva.
>>>
>>> Oh, no! We also have mappings not backed by a BO at all! For example 
>>> for partial resident textures or data routing to internal hw etc...
>>>
>>> You can't be sure that a mapping is backed by a BO at all.
>>
>> I fully agree, that's why I wrote "the common case should be that in a 
>> VA space at least every mapping *being backed by a BO* is represented 
>> by a struct drm_gpuva".
>>
>> Mappings not being backed by an actual BO would not be linked to a GEM 
>> of course.
>>
>>>
>>>>
>>>>>
>>>>> Instead I suggest to have a separate structure for mappings in a VA 
>>>>> space which driver can then add to their GEM objects or whatever 
>>>>> they want to map into their VMs.
>>>>
>>>> Which kind of separate structure for mappings? Another one analogous 
>>>> to struct drm_gpuva?
>>>
>>> Well similar to what amdgpu uses BO -> one structure for each 
>>> combination of BO and VM -> mappings inside that VM
>>
>> As explained above, I think that's exactly what the GPUVA manager 
>> does, just in another order:
>>
>> BO has list of mappings, mappings have pointer to VM, VM has list (or 
>> actually a maple tree) of mappings.
>>
>> You see any advantages or disadvantages of either order of 
>> relationships? For me it looks like it doesn't really matter which one 
>> to pick.
>>
>> - Danilo
>>
>>>
>>> Regards,
>>> Christian.
>>>
>>>>
>>>> - Danilo
>>>>
>>>>>
>>>>> Regards,
>>>>> Christian.
>>>>>
>>>>>
>>>>
>>>
>>
>