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. To process the second request, userspace uses VDUSE_IOTLB_GET_FD ioctl to query the new iova region and map a new region (0 ~ 8192). Then userspace needs to traverse the list of iova regions and unmap the old region (0 ~ 4096). Looks like this is a little complex. Thanks, Yongji
