`vfio_dma_map` should be unsafe but isn't

[DemiMarie]

The function vfio_dma_map takes a u64 and passes it to a kernel function that treats it as a pointer and accesses the virtual address space it points to. Such a function must be unsafe, but this function currently is safe.

I'm not sure how to provide an unopinionated safe wrapper for this API. An opinionated option would be to have a type that allocates the memory with mmap, exposes an API to call vfio_dma_map initialization, and (if the memory had been mapped) calls vfio_dma_unmap in its Drop::drop. I'm not sure if this is sufficient for Cloud Hypervisor, though.

[alyssais]

Sounds like marking the unsafe function as unsafe would be a good first step. Then callers can at least know that they're using it safely.

(It's been fine to do this for other rust-vmm crates btw, in case you were worried about the API breakage, e.g. rust-vmm/kvm@47665cb)