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. > > Userfaultfd is an fd because one program controls another. Is mempinfd like this? We use mempinfd like: (see patch 2/2) fd = mempinfd(); va = malloc(size); struct mem_pin_address addr; addr.va = va; addr.size = size; ioctl(fd, MEM_CMD_PIN, &addr); ioctl(fd, MEM_CMD_UNPIN, &addr); close(fd); Best, Zhou > . >
