Re: [Nouveau] [PATCH drm-next 05/14] drm/nouveau: new VM_BIND uapi interfaces

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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.





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