On Tue, Apr 16, 2013 at 3:03 AM, Simon Jeons <simon.jeons@xxxxxxxxx> wrote:
As i said this is for pre-filling already present entry, ie pte that are present with a valid page (no special bit set). This is an optimization so that the GPU can pre-fill its tlb without having to take any mmap_sem. Hope is that in most common case this will be enough, but in some case you will have to go through the lengthy non fast gup.
Cheers,Hi Jerome,get_user_pages_fast just get page reference count instead of populate the pte to page table, correct? Then how can GPU driver use iommu to access the page?
On 02/08/2013 11:21 PM, Jerome Glisse wrote:
On Fri, Feb 8, 2013 at 6:18 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.
Following is a longer motivation and explanation on the technology
presented:
Many application developers would like to be able to be able to communicate
directly with the hardware from the userspace.
Use cases for that includes high performance networking API such as
InfiniBand, RoCE and iWarp and interfacing with GPUs.
Currently, if the user space application wants to share system memory with
the hardware device, the kernel component must pin the memory pages in RAM,
using get_user_pages.
This is a hurdle, as it usually makes large portions the application memory
unmovable. This pinning also makes the user space development model very
complicated – one needs to register memory before using it for communication
with the hardware.
We use the mmu-notifiers [1] mechanism to inform the hardware when the
mapping of a page is changed. If the hardware tries to access a page which
is not yet mapped for the hardware, it requests a resolution for the page
address from the kernel.
This mechanism allows the hardware to access the entire address space of the
user application, without pinning even a single page.
We would like to use the LSF/MM forum opportunity to discuss open issues we
have for further development, such as:
-Allowing the hardware to perform page table walk, similar to
get_user_pages_fast to resolve user pages that are already in RAM.
As i said this is for pre-filling already present entry, ie pte that are present with a valid page (no special bit set). This is an optimization so that the GPU can pre-fill its tlb without having to take any mmap_sem. Hope is that in most common case this will be enough, but in some case you will have to go through the lengthy non fast gup.
Jerome