HOST_WRITE useful or validation layers wrong?

Question

In the specs about HOST_WRITE_BIT it is write :

For host writes to be seen by subsequent command buffer operations, a pipeline barrier from a source of VK_ACCESS_HOST_WRITE_BIT and VK_PIPELINE_STAGE_HOST_BIT to a destination of the relevant device pipeline stages and access types must be performed. Note that such a barrier is performed implicitly upon each command buffer submission, so an explicit barrier is only rarely needed

However, when you transition (through a vkCmdPipelineBarrier (so in a commandBuffer)) one image from PREINITIALIZED layout with a srcAccessMask to 0 instead of HOST_WRITE_BIT, you get an error :

validation layer: Source AccessMask 0 [None] must have required access bit 16384 [VK_ACCESS_HOST_WRITE_BIT] when layout is VK_IMAGE_LAYOUT_PREINITIALIZED, unless the app has previously added a barrier for this transition.

Is there an error from the specs? From the validation layers? Is the barrier we are talking about is a pure execution barrier and not a memory one? Am I missing something?

Answer 1

My private opinion is: validation layers bug.

It simply checks layout vs access flags and does not seem to be aware of this corner case: https://github.com/KhronosGroup/Vulkan-LoaderAndValidationLayers/blob/master/layers/core_validation.cpp#L9005

There's also a case when you reconsider and do not host write to the VK_IMAGE_LAYOUT_PREINITIALIZED Image at all (and thus need no barrier), isn't it?

I believe the layer message is a WARNING, not ERROR. It may mean it is "only" a heuristic and some false positives are expected (until they improve the layers, which seem posible, but not so trivial for this case).

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They even only lately corrected possibility for 0 access flags for the presentation, so it is not far fetched they would (with similar mind) forget something like that in the layers.

I would report an Issue there. They do not bite and worse that can happen is that some Khronos insider more knowledgable than me will explain why you are wrong.

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That being said, perhaps the VK_PIPELINE_STAGE_HOST_BIT is unnecessary too (and TOP should suffice)?