Semantics of dma_{map,pin,unmap}_guest_page

[Julian Stecklina]

Hello,

as you know we are writing a hypervisor backend for i915/gvt. We were wondering
about the semantics of dma_map_guest_page, dma_pin_guest_page, and
dma_unmap_guest_page from intel_gvt_ops.

My current understanding is this: Map creates a new DMA mapping with a reference
count of 1. Pin increases the reference count by one. Unmap decreases the
reference count by 1 and if it reaches zero, removes the DMA mapping. Pretty
straight forward.

I would also expect that after I call `intel_gvt_ops->vgpu_destroy`, all DMA
mappings are released by the mediator with the appropriate number of unmap
calls. This doesn't seem to be the case, as I see many DMA mappings that are
still alive after the vGPU is destroyed.

At this point, I'm a bit unsure what to do with these mappings, because they
might still be in use. So the options are to either free them (and risk memory
corruption) or keep them around leak memory.

Do I have a flaw in my assumptions or is it expected behavior to clean up some
mappings that still have a reference count >0 after the vGPU is destroyed?

Thanks,
Julian