On Fri, Mar 5, 2021 at 3:27 PM Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > On 2021/3/5 3:13 下午, Yongji Xie wrote: > > On Fri, Mar 5, 2021 at 2:52 PM Jason Wang <jasowang@xxxxxxxxxx> wrote: > >> > >> On 2021/3/5 2:15 下午, 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, Yongji
