Message-ID: <2f502150-c1f8-615c-66d9-c3fb59b8c409@redhat.com>
Date:   Thu, 22 Jun 2023 17:07:11 +0200
From:   Danilo Krummrich <dakr@...hat.com>
To:     Christian König <christian.koenig@....com>,
        airlied@...il.com, daniel@...ll.ch, tzimmermann@...e.de,
        mripard@...nel.org, corbet@....net, bskeggs@...hat.com,
        Liam.Howlett@...cle.com, matthew.brost@...el.com,
        boris.brezillon@...labora.com, alexdeucher@...il.com,
        ogabbay@...nel.org, bagasdotme@...il.com, willy@...radead.org,
        jason@...kstrand.net
Cc:     dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-doc@...r.kernel.org, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Donald Robson <donald.robson@...tec.com>,
        Dave Airlie <airlied@...hat.com>
Subject: Re: [PATCH drm-next v5 03/14] drm: manager to keep track of GPUs VA
 mappings

On 6/22/23 17:04, Danilo Krummrich wrote:
> 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):

*than with a BO having a list of VMs:

> 
> 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.
>>>>>>
>>>>>>
>>>>>
>>>>
>>>
>>

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