[brooksbp]

> is there any reasonable prospect of keeping one's GPU from being able to read and write to literally anywhere in physical memory?

This is the purpose of an IOMMU.

> I.e., a practical way a kernel and driver might be able to forward to the GPU only commands and shaders that can access only your process memory, and nobody else's, and your process's pixels, and no other process's pixels, when they live in GPU RAM?

So IOMMU and the GPU's MMU.

> For all I know, this is the norm for all GPUs

What?

> but I wonder why it is hard, then, for VMs to share a GPU.

Engineering is hard.