Jason Wang
2021-Mar-08 03:52 UTC
[RFC v4 06/11] vduse: Implement an MMU-based IOMMU driver
On 2021/3/8 11:45 ??, Yongji Xie wrote:> On Mon, Mar 8, 2021 at 11:17 AM Jason Wang <jasowang at redhat.com> wrote: >> >> 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, >>> 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].It should still be [0, 64M). Do you see any issue? Thanks> > Thanks, > Yongji >