Jason Wang
2021-Mar-08 03:17 UTC
[RFC v4 06/11] vduse: Implement an MMU-based IOMMU driver
On 2021/3/5 3:59 ??, Yongji Xie wrote:> On Fri, Mar 5, 2021 at 3:27 PM Jason Wang <jasowang at redhat.com> wrote: >> >> On 2021/3/5 3:13 ??, Yongji Xie wrote: >>> On Fri, Mar 5, 2021 at 2:52 PM Jason Wang <jasowang at redhat.com> wrote: >>>> On 2021/3/5 2:15 ??, Yongji Xie wrote: >>>> >>>> Sorry if I've asked this before. >>>> >>>> But what's the reason for maintaing a dedicated IOTLB here? I think we >>>> could reuse vduse_dev->iommu since the device can not be used by both >>>> virtio and vhost in the same time or use vduse_iova_domain->iotlb for >>>> set_map(). >>>> >>>> The main difference between domain->iotlb and dev->iotlb is the way to >>>> deal with bounce buffer. In the domain->iotlb case, bounce buffer >>>> needs to be mapped each DMA transfer because we need to get the bounce >>>> pages by an IOVA during DMA unmapping. In the dev->iotlb case, bounce >>>> buffer only needs to be mapped once during initialization, which will >>>> be used to tell userspace how to do mmap(). >>>> >>>> Also, since vhost IOTLB support per mapping token (opauqe), can we use >>>> that instead of the bounce_pages *? >>>> >>>> Sorry, I didn't get you here. Which value do you mean to store in the >>>> opaque pointer? >>>> >>>> So I would like to have a way to use a single IOTLB for manage all kinds >>>> of mappings. Two possible ideas: >>>> >>>> 1) map bounce page one by one in vduse_dev_map_page(), in >>>> VDUSE_IOTLB_GET_FD, try to merge the result if we had the same fd. Then >>>> for bounce pages, userspace still only need to map it once and we can >>>> maintain the actual mapping by storing the page or pa in the opaque >>>> field of IOTLB entry. >>>> >>>> Looks like userspace still needs to unmap the old region and map a new >>>> region (size is changed) with the fd in each VDUSE_IOTLB_GET_FD ioctl. >>>> >>>> >>>> I don't get here. Can you give an example? >>>> >>> For example, userspace needs to process two I/O requests (one page per >>> request). To process the first request, userspace uses >>> VDUSE_IOTLB_GET_FD ioctl to query the iova region (0 ~ 4096) and mmap >>> it. >> >> I think in this case we should let VDUSE_IOTLB_GET_FD return the maximum >> range as far as they are backed by the same fd. >> > But now the bounce page is mapped one by one. The second page (4096 ~ > 8192) might not be mapped when userspace is processing the first > request. So the maximum range is 0 ~ 4096 at that time. > > Thanks, > YongjiA question, if I read the code correctly, VDUSE_IOTLB_GET_FD will return the whole bounce map range which is setup in vduse_dev_map_page()? So my understanding is that usersapce may choose to map all its range via mmap(). So if we 'map' bounce page one by one in vduse_dev_map_page(). (Here 'map' means using multiple itree entries instead of a single one). Then in the VDUSE_IOTLB_GET_FD we can keep traversing itree (dev->iommu) until the range is backed by a different file. With this, there's no userspace visible changes and there's no need for the domain->iotlb? Thanks>