Re: Feature Request: Ability to decode bus/dma address back into physical address

[Jerome Glisse <j.glisse-Re5JQEeQqe8AvxtiuMwx3w@public.gmane.org>]

On Tue, Aug 01, 2017 at 06:07:48AM -0400, Tom St Denis wrote:
> Hi,
> 
> We're working on a user space debugger for AMDGPU devices and are trying to
> figure out a "proper" way of taking mapped pages and converting them back to
> physical addresses so the debugger can read memory that was sent to the GPU.
> 
> The debugger largely operates at arms length from the application being
> debugged so it has no knowledge of the buffers other than which PCI device
> mapped it and the mapped address.

There is your issue, you should reconsider your design. You should add a
debuging/tracing API to the amdgpu kernel driver that allow one process
to snoop on another process buffers. It would be a lot more realistic user
space interface.

> 
> We also use the debugger to read back kernel programmed buffers (ring
> buffers and the like) so we really need a kernel level solution.

Not going to happen, this would be like /dev/mem on steroid and a big
security risk.

> 
> As a prototype I put a trace point in the AMDGPU driver when pci_map_page()
> is called which preserves the physical and dma address and that works but
> obviously is a bit of a hack and doesn't work if pages are mapped before the
> trace is enabled.
> 
> Ideally, some form of debugfs interface would be nice.
> 
> Is there any sort of interface already I can take advantage of?  I've tried
> enabling the map/unmap tracepoints before loading amdgpu and it produced no
> traffic in the trace file.

I think you need to reconsider how to achieve your goal. It is a lot more
sensefull to add new API to amdgpu driver than asking kernel to provide you
with access to random physical memory.

Jérôme