vm binding interfaces and parallel with mmap

[Bas Nieuwenhuizen]

On Mon, Aug 22, 2022 at 9:28 AM Dave Airlie <airlied at gmail.com> wrote:
>
> On Mon, 22 Aug 2022 at 17:05, Dave Airlie <airlied at gmail.com> wrote:
> >
> > Hey,
> >
> > I've just been looking at the vm bind type interfaces and wanted to at
> > least document how we think the unmapping API should work. I know I've
> > talked on irc before about this, but wanted to solidify things a bit
> > more around what is required vs what is a nice to have.
> >
> > My main concerns/thoughts are around the unbind interfaces and how
> > close to munmap they should be.
> >
> > I think the mapping operation is mostly consistent
> > MAP(bo handle, offset into bo, range, VM offset, VM flags)
> > which puts the range inside to bo at the offset in the current VM
> > (maybe take an optional vm_id).
> >
> > now the simplest unmap I can see if one that parallel munmap
> > UNMAP(vmaddr, range);
> >
> > But it begs the question on then how much the kernel needs to deal
> > with here, if we support random vmaddr,range then we really need to be
> > able to do everything munmap does for CPU VMA, which means splitting
> > ranges, joining ranges etc.
> >
> > like
> > MAP(1, 0, 0x8000, 0xc0000)
> > UNMAP(0xc1000, 0x1000)
> > should that be possible?
> >
> > Do we have any API usage (across Vulkan/CL/CUDA/ROCm etc) that
> > requires this sort of control, or should we be fine with only
> > unmapping objects exactly like how they were mapped in the first
> > place, and not have any splitting/joining?

Vulkan allows for this, though I haven't checked to what extent apps use it.

We could technically split all mapping/unmapping to be per single tile
in the userspace driver, which avoids the need for splitting/merging,
but that could very much be a pessimization.


>
> I suppose it also asks the question around paralleling
>
> fd = open()
> ptr = mmap(fd,)
> close(fd)
> the mapping is still valid.
>
> I suppose our equiv is
> handle = bo_alloc()
> gpu_addr = vm_bind(handle,)
> gem_close(handle)
> is the gpu_addr still valid does the VM hold a reference on the kernel
> bo internally.

For Vulkan it looks like this is undefined and the above is not necessary:

"It is important to note that freeing a VkDeviceMemory object with
vkFreeMemory will not cause resources (or resource regions) bound to
the memory object to become unbound. Applications must not access
resources bound to memory that has been freed."
(32.7.6)


>
> Dave.
> >
> > Dave.