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 :) thanks, greg k-h
