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

[Felix Kuehling <felix.kuehling@amd.com>]

On 2018-03-10 10:01 AM, Christian KA?nig 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.

Regards,
A  Felix