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

[Jerome Glisse <jglisse@redhat.com>]

On Mon, Mar 12, 2018 at 02:28:42PM -0400, Felix Kuehling wrote:
> On 2018-03-10 10:01 AM, Christian Konig wrote:
> >> To accomodate those we need to
> >> create a "hole" inside the process address space. This patchset have
> >> a hack for that (patch 13 HACK FOR HMM AREA), it reserves a range of
> >> device file offset so that process can mmap this range with PROT_NONE
> >> to create a hole (process must make sure the hole is below 1 << 40).
> >> I feel un-easy of doing it this way but maybe it is ok with other
> >> folks.
> >
> > Well we have essentially the same problem with pre gfx9 AMD hardware.
> > Felix might have some advise how it was solved for HSA. 
> 
> For pre-gfx9 hardware we reserve address space in user mode using a big
> mmap PROT_NONE call at application start. Then we manage the address
> space in user mode and use MAP_FIXED to map buffers at specific
> addresses within the reserved range.
> 
> The big address space reservation causes issues for some debugging tools
> (clang-sanitizer was mentioned to me), so with gfx9 we're going to get
> rid of this address space reservation.

What do you need those mapping for ? What kind of object (pm4 packet
command buffer, GPU semaphore | fence, ...) ? Kernel private object ?
On nv we need it for the main command buffer ring which we do not want
to expose to application.

Thus for nv gpu we need kernel to monitor this PROT_NONE region to make
sure that i never got unmapped, resize, move ... this is me fearing a
rogue userspace that munmap and try to abuse some bug in SVM/GPU driver
to abuse object map behind those fix mapping.

Cheers,
Jerome