Message-ID: <6566870d-6256-8eef-5879-cb13711e4bed@redhat.com>
Date:   Thu, 19 Jan 2023 05:04:32 +0100
From:   Danilo Krummrich <dakr@...hat.com>
To:     Christian König <christian.koenig@....com>,
        Dave Airlie <airlied@...il.com>,
        Alex Deucher <alexdeucher@...il.com>
Cc:     tzimmermann@...e.de, corbet@....net, nouveau@...ts.freedesktop.org,
        dri-devel@...ts.freedesktop.org, linux-doc@...r.kernel.org,
        linux-kernel@...r.kernel.org, bskeggs@...hat.com,
        jason@...kstrand.net, airlied@...hat.com
Subject: Re: [PATCH drm-next 00/14] [RFC] DRM GPUVA Manager & Nouveau VM_BIND
 UAPI

On 1/18/23 20:48, Christian König wrote:
> Am 18.01.23 um 20:17 schrieb Dave Airlie:
>> On Thu, 19 Jan 2023 at 02:54, Alex Deucher <alexdeucher@...il.com> wrote:
>>> On Wed, Jan 18, 2023 at 11:50 AM Danilo Krummrich <dakr@...hat.com> 
>>> wrote:
>>>>
>>>>
>>>> On 1/18/23 17:30, Alex Deucher wrote:
>>>>> On Wed, Jan 18, 2023 at 11:19 AM Danilo Krummrich <dakr@...hat.com> 
>>>>> wrote:
>>>>>> On 1/18/23 16:37, Christian König wrote:
>>>>>>> Am 18.01.23 um 16:34 schrieb Danilo Krummrich:
>>>>>>>> Hi Christian,
>>>>>>>>
>>>>>>>> On 1/18/23 09:53, Christian König wrote:
>>>>>>>>> Am 18.01.23 um 07:12 schrieb Danilo Krummrich:
>>>>>>>>>> This patch series provides a new UAPI for the Nouveau driver in
>>>>>>>>>> order to
>>>>>>>>>> support Vulkan features, such as sparse bindings and sparse 
>>>>>>>>>> residency.
>>>>>>>>>>
>>>>>>>>>> Furthermore, with the DRM GPUVA manager it provides a new DRM 
>>>>>>>>>> core
>>>>>>>>>> feature to
>>>>>>>>>> keep track of GPU virtual address (VA) mappings in a more 
>>>>>>>>>> generic way.
>>>>>>>>>>
>>>>>>>>>> The DRM GPUVA manager is indented to help drivers implement
>>>>>>>>>> userspace-manageable
>>>>>>>>>> GPU VA spaces in reference to the Vulkan API. In order to achieve
>>>>>>>>>> this goal it
>>>>>>>>>> serves the following purposes in this context.
>>>>>>>>>>
>>>>>>>>>>        1) Provide a dedicated range allocator to track GPU VA
>>>>>>>>>> allocations and
>>>>>>>>>>           mappings, making use of the drm_mm range allocator.
>>>>>>>>> This means that the ranges are allocated by the kernel? If yes 
>>>>>>>>> that's
>>>>>>>>> a really really bad idea.
>>>>>>>> No, it's just for keeping track of the ranges userspace has 
>>>>>>>> allocated.
>>>>>>> Ok, that makes more sense.
>>>>>>>
>>>>>>> So basically you have an IOCTL which asks kernel for a free 
>>>>>>> range? Or
>>>>>>> what exactly is the drm_mm used for here?
>>>>>> Not even that, userspace provides both the base address and the 
>>>>>> range,
>>>>>> the kernel really just keeps track of things. Though, writing a 
>>>>>> UAPI on
>>>>>> top of the GPUVA manager asking for a free range instead would be
>>>>>> possible by just adding the corresponding wrapper functions to get a
>>>>>> free hole.
>>>>>>
>>>>>> Currently, and that's what I think I read out of your question, 
>>>>>> the main
>>>>>> benefit of using drm_mm over simply stuffing the entries into a 
>>>>>> list or
>>>>>> something boils down to easier collision detection and iterating
>>>>>> sub-ranges of the whole VA space.
>>>>> Why not just do this in userspace?  We have a range manager in
>>>>> libdrm_amdgpu that you could lift out into libdrm or some other
>>>>> helper.
>>>> The kernel still needs to keep track of the mappings within the various
>>>> VA spaces, e.g. it silently needs to unmap mappings that are backed by
>>>> BOs that get evicted and remap them once they're validated (or swapped
>>>> back in).
>>> Ok, you are just using this for maintaining the GPU VM space in the 
>>> kernel.
>>>
>> Yes the idea behind having common code wrapping drm_mm for this is to
>> allow us to make the rules consistent across drivers.
>>
>> Userspace (generally Vulkan, some compute) has interfaces that pretty
>> much dictate a lot of how VMA tracking works, esp around lifetimes,
>> sparse mappings and splitting/merging underlying page tables, I'd
>> really like this to be more consistent across drivers, because already
>> I think we've seen with freedreno some divergence from amdgpu and we
>> also have i915/xe to deal with. I'd like to at least have one place
>> that we can say this is how it should work, since this is something
>> that *should* be consistent across drivers mostly, as it is more about
>> how the uapi is exposed.
> 
> That's a really good idea, but the implementation with drm_mm won't work 
> like that.
> 
> We have Vulkan applications which use the sparse feature to create 
> literally millions of mappings. That's why I have fine tuned the mapping 
> structure in amdgpu down to ~80 bytes IIRC and save every CPU cycle 
> possible in the handling of that.

That's a valuable information. Can you recommend such an application for 
testing / benchmarking?

Your optimization effort sounds great. May it be worth thinking about 
generalizing your approach by itself and stacking the drm_gpuva_manager 
on top of it?

> 
> A drm_mm_node is more in the range of ~200 bytes and certainly not 
> suitable for this kind of job.
> 
> I strongly suggest to rather use a good bunch of the amdgpu VM code as 
> blueprint for the common infrastructure.

I will definitely have look.

> 
> Regards,
> Christian.
> 
>>
>> Dave.
> 

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