linux-kernel - Re: [PATCH drm-misc-next v8 09/12] drm/gpuvm: reference count drm

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Message-ID: <71a50cac-3253-480f-adff-da475738ae6b@amd.com>
Date:   Mon, 13 Nov 2023 08:22:18 +0100
From:   Christian König <christian.koenig@....com>
To:     Danilo Krummrich <dakr@...hat.com>,
        Thomas Hellström 
        <thomas.hellstrom@...ux.intel.com>
Cc:     airlied@...il.com, daniel@...ll.ch, matthew.brost@...el.com,
        sarah.walker@...tec.com, donald.robson@...tec.com,
        boris.brezillon@...labora.com, faith@...strand.net,
        dri-devel@...ts.freedesktop.org, nouveau@...ts.freedesktop.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH drm-misc-next v8 09/12] drm/gpuvm: reference count
 drm_gpuvm structures

Am 10.11.23 um 17:57 schrieb Danilo Krummrich:
> On 11/10/23 09:50, Christian König wrote:
>> [SNIP]
>>>>
>>>>>
>>>>> Another issue Christian brought up is that something intended to 
>>>>> be embeddable (a base class) shouldn't really have its own 
>>>>> refcount. I think that's a valid point. If you at some point need 
>>>>> to derive from multiple such structs each having its own refcount, 
>>>>> things will start to get weird. One way to resolve that would be 
>>>>> to have the driver's subclass provide get() and put() ops, and 
>>>>> export a destructor for the base-class, rather than to have the 
>>>>> base-class provide the refcount and a destructor  ops.
>>>
>>> GPUVM simply follows the same pattern we have with drm_gem_objects. 
>>> And I think it makes
>>> sense. Why would we want to embed two struct drm_gpuvm in a single 
>>> driver structure?
>>
>> Because you need one drm_gpuvm structure for each application using 
>> the driver? Or am I missing something?
>
> Right, *one*, but not more than one. Wasn't that the concern? Maybe I 
> misunderstood something. :)

Well, there is the use case of native context with XEN/KVM. In that 
situation QEMU opens tons of driver file descriptors on behalves of the 
virtual environment clients.

In this use case you have many drm_gpuvm instances for a single 
application. So you can't assume that you only have one VM per PID/TGID 
or something like that.

AMD already made that mistake with KFD and I strongly suggest not to 
repeat it :)

Regards,
Christian.