[Nouveau] [RFC PATCH 00/13] SVM (share virtual memory) with HMM in nouveau

[Matthew Wilcox]

On Mon, Mar 12, 2018 at 11:14:47PM -0700, John Hubbard wrote:
> Yes, on NVIDIA GPUs, the Host/FIFO unit is limited to 40-bit addresses, so
> things such as the following need to be below (1 << 40), and also accessible 
> to both CPU (user space) and GPU hardware. 
>     -- command buffers (CPU user space driver fills them, GPU consumes them), 
>     -- semaphores (here, a GPU-centric term, rather than OS-type: these are
>        memory locations that, for example, the GPU hardware might write to, in
>        order to indicate work completion; there are other uses as well), 
>     -- a few other things most likely (this is not a complete list).

Is that a 40-bit virtual address limit or physical address limit?  I'm
no longer sure who is addressing what memory through what mechanism ;-)