linux-kernel - Re: [Nouveau] [PATCH drm-next 05/14] drm/nouveau: new VM

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Message-ID: <4439c44b-cbd0-7160-da3d-e78f6aeeec77@redhat.com>
Date:   Mon, 6 Feb 2023 19:20:52 +0100
From:   Danilo Krummrich <dakr@...hat.com>
To:     Christian König <christian.koenig@....com>,
        Dave Airlie <airlied@...il.com>
Cc:     Matthew Brost <matthew.brost@...el.com>, daniel@...ll.ch,
        corbet@....net, dri-devel@...ts.freedesktop.org,
        linux-doc@...r.kernel.org, linux-kernel@...r.kernel.org,
        mripard@...nel.org, bskeggs@...hat.com, jason@...kstrand.net,
        nouveau@...ts.freedesktop.org, airlied@...hat.com
Subject: Re: [Nouveau] [PATCH drm-next 05/14] drm/nouveau: new VM_BIND uapi
 interfaces

On 2/6/23 17:14, Christian König wrote:
> Concentrating this discussion on a very big misunderstanding first.
> 
> Am 06.02.23 um 14:27 schrieb Danilo Krummrich:
>> [SNIP]
>> My understanding is that userspace is fully responsible on the parts 
>> of the GPU VA space it owns. This means that userspace needs to take 
>> care to *not* ask the kernel to modify mappings that are in use 
>> currently.
> 
> This is a completely wrong assumption! Take a look at what games like 
> Forza Horizzon are doing.
> 
> Basically that game allocates a very big sparse area and fills it with 
> pages from BOs while shaders are accessing it. And yes, as far as I know 
> this is completely valid behavior.

I also think this is valid behavior. That's not the problem I'm trying 
to describe. In this case userspace modifies the VA space 
*intentionally* while shaders are accessing it, because it knows that 
the shaders can deal with reading 0s.

Just to have it all in place, the example I gave was:
  - two virtually contiguous buffers A and B
  - binding 1 mapped to A with BO offset 0
  - binding 2 mapped to B with BO offset length(A)

What I did not mention both A and B aren't sparse buffers in this 
example, although it probably doesn't matter too much.

Since the conditions to do so are given, we merge binding 1 and binding 
2 right at the time when binding 2 is requested. To do so a driver might 
unmap binding 1 for a very short period of time (e.g. to (re-)map the 
freshly merged binding with a different page size if possible).

 From userspace perspective buffer A is ready to use before applying 
binding 2 to buffer B, hence it would be illegal to touch binding 1 
again when userspace asks the kernel to map binding 2 to buffer B.

Besides that I think there is no point in merging between buffers anyway 
because we'd end up splitting such a merged mapping anyway later on when 
one of the two buffers is destroyed.

Also, I think the same applies to sparse buffers as well, a mapping 
within A isn't expected to be re-mapped just because something is mapped 
to B.

However, in this context I start wondering if re-mapping in the context 
of merge and split is allowed at all, even within the same sparse buffer 
(and even with a separate page table for sparse mappings as described in 
my last mail; shaders would never fault).

> 
> So you need to be able to handle this case anyway and the approach with 
> the regions won't help you at all preventing that.
> 
> Regards,
> Christian.
>