On Mon, Mar 8, 2021 at 11:17 AM Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > On 2021/3/5 3:59 下午, Yongji Xie wrote: > > 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 > > > A 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(). > Yes. > 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? > In this case, I wonder what range can be obtained if userspace calls VDUSE_IOTLB_GET_FD when the first I/O (e.g. 4K) occurs. [0, 4K] or [0, 64M]? In current implementation, userspace will map [0, 64M]. Thanks, Yongji
