On Tue, Feb 09, 2021 at 07:58:15PM +0800, Zhou Wang wrote: > On 2021/2/9 17:37, Greg KH wrote: > > On Tue, Feb 09, 2021 at 05:17:46PM +0800, Zhou Wang wrote: > >> On 2021/2/8 6:02, Andy Lutomirski wrote: > >>> > >>> > >>>> On Feb 7, 2021, at 12:31 AM, Zhou Wang <wangzhou1@xxxxxxxxxxxxx> wrote: > >>>> > >>>> SVA(share virtual address) offers a way for device to share process virtual > >>>> address space safely, which makes more convenient for user space device > >>>> driver coding. However, IO page faults may happen when doing DMA > >>>> operations. As the latency of IO page fault is relatively big, DMA > >>>> performance will be affected severely when there are IO page faults. > >>>> From a long term view, DMA performance will be not stable. > >>>> > >>>> In high-performance I/O cases, accelerators might want to perform > >>>> I/O on a memory without IO page faults which can result in dramatically > >>>> increased latency. Current memory related APIs could not achieve this > >>>> requirement, e.g. mlock can only avoid memory to swap to backup device, > >>>> page migration can still trigger IO page fault. > >>>> > >>>> Various drivers working under traditional non-SVA mode are using > >>>> their own specific ioctl to do pin. Such ioctl can be seen in v4l2, > >>>> gpu, infiniband, media, vfio, etc. Drivers are usually doing dma > >>>> mapping while doing pin. > >>>> > >>>> But, in SVA mode, pin could be a common need which isn't necessarily > >>>> bound with any drivers, and neither is dma mapping needed by drivers > >>>> since devices are using the virtual address of CPU. Thus, It is better > >>>> to introduce a new common syscall for it. > >>>> > >>>> This patch leverages the design of userfaultfd and adds mempinfd for pin > >>>> to avoid messing up mm_struct. A fd will be got by mempinfd, then user > >>>> space can do pin/unpin pages by ioctls of this fd, all pinned pages under > >>>> one file will be unpinned in file release process. Like pin page cases in > >>>> other places, can_do_mlock is used to check permission and input > >>>> parameters. > >>> > >>> > >>> Can you document what the syscall does? > >> > >> Will add related document in Documentation/vm. > > > > A manpage is always good, and will be required eventually :) > > manpage is maintained in another repo. Do you mean add a manpage > patch in this series? It's good to show how it will be used, don't you think? thanks, greg k-h
