On 2021/2/9 20:01, Greg KH wrote: > 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? Agree, will add it in next version. Thanks, Zhou > > thanks, > > greg k-h > > . >
