Support for importing the external memory

[Agrael1]

Hi, I have recently been working on Cef library, and since I use VMA I would like to have a way to import external memory.
Right now it works by creating a pool and allocating from that pool, but since Cef requires to load the memory each time, I wonder if the process can be streamlined.

My idea is to make a set of import functions that just return the allocation and all the data about it without creating pools, which are still allocated in full and then need to be separately cleaned up. This reduces overhead of import operations and provide uniform interface to work with.

I can work on such extension the same way I did for export.

The same can be done for D3DMA.

[sawickiap]

What Vulkan functions do you propose to encapsulate and what interface for VMA do you propose to add regarding external memory import? Any additional feedback and contribution is welcome.

[Agrael1]

@sawickiap I propose to encapsulate VkAllocateMemory for the case when the external memory is imported. Right now there is no way to add pNext to allocation aside from memory pools. I would like to do that in the same way as dedicated allocations work, similarly to the D3DMA ability to allocate blocks with custom memory without pool.

Interface proposal:
For now I'd go with only one new function vmaImportExternalMemoryWin32() which accepts:

1. VkExternalMemoryHandleTypeFlagBits to load correct handle (DX11, DX12, Win32 opaque + KMT handles)
2. HANDLE to load from
3. uint64_t import size. This one is optional for D3D12 and D3D11 resources and can be set to 0

Since most of the memory requirements are imported with the handle itself and must be queried with vkGetMemoryWin32HandlePropertiesKHR it can be encapsulated within the function.

The resulting allocation must keep the provided handle and use CloseHandle on deallocation if provided bits require freeing the handle. Now for global shared handles:

You could potentially query the handle with OpenFileMappingW and then pass it forward. But maybe adding a union argument would be better.

[sawickiap]

Thank you for this suggestion. I will consider adding this feature and I will let you know once I have some prototype code prepared on a branch, for your feedback.