Hi Jerome,
On 02/10/2013 12:29 AM, Jerome Glisse wrote:
On Sat, Feb 9, 2013 at 1:05 AM, Michel Lespinasse <walken@xxxxxxxxxx> wrote:
On Fri, Feb 8, 2013 at 3:18 AM, Shachar Raindel <raindel@xxxxxxxxxxxx> wrote:
Hi,
We would like to present a reference implementation for safely sharing
memory pages from user space with the hardware, without pinning.
We will be happy to hear the community feedback on our prototype
implementation, and suggestions for future improvements.
We would also like to discuss adding features to the core MM subsystem to
assist hardware access to user memory without pinning.
This sounds kinda scary TBH; however I do understand the need for such
technology.
I think one issue is that many MM developers are insufficiently aware
of such developments; having a technology presentation would probably
help there; but traditionally LSF/MM sessions are more interactive
between developers who are already quite familiar with the technology.
I think it would help if you could send in advance a detailed
presentation of the problem and the proposed solutions (and then what
they require of the MM layer) so people can be better prepared.
And first I'd like to ask, aren't IOMMUs supposed to already largely
solve this problem ? (probably a dumb question, but that just tells
you how much you need to explain :)
For GPU the motivation is three fold. With the advance of GPU compute
and also with newer graphic program we see a massive increase in GPU
memory consumption. We easily can reach buffer that are bigger than
1gbytes. So the first motivation is to directly use the memory the
user allocated through malloc in the GPU this avoid copying 1gbytes of
data with the cpu to the gpu buffer. The second and mostly important
to GPU compute is the use of GPU seamlessly with the CPU, in order to
achieve this you want the programmer to have a single address space on
the CPU and GPU. So that the same address point to the same object on
GPU as on the CPU. This would also be a tremendous cleaner design from
driver point of view toward memory management.
When GPU will comsume memory?
The userspace process like mplayer will have video datas and GPU will
play this datas and use memory of mplayer since these video datas load
in mplayer process's address space? So GPU codes will call gup to take a
reference of memory? Please correct me if my understanding is wrong. ;-)
And last, the most important, with such big buffer (>1gbytes) the
memory pinning is becoming way to expensive and also drastically
reduce the freedom of the mm to free page for other process. Most of
the time a small window (every thing is relative the window can be >
100mbytes not so small :)) of the object will be in use by the
hardware. The hardware pagefault support would avoid the necessity to
pin memory and thus offer greater flexibility. At the same time the
driver wants to avoid page fault as much as possible this is why i
would like to be able to give hint to the mm about range of address it
should avoid freeing page (swapping them out).
The iommu was designed with other goals, which were first isolate
device from one another and restrict device access to allowed memory.
Second allow to remap address that are above device address space
limit. Lot of device can only address 24bit or 32bit of memory and
with computer with several gbytes of memory suddenly lot of the page
become unreachable to the hardware. The iommu allow to work around
this by remapping those high page into address that the hardware can
reach.
The hardware page fault support is a new feature of iommu designed to
help the os and driver to reduce memory pinning and also share address
space. Thought i am sure there are other motivations that i am not
even aware off or would think off.
Btw i won't be at LSF/MM so a free good beer (or other beverage) on me
to whoever takes note on this subject in next conf we run into each
others.
Cheers,
Jerome
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