Re: Re: [RFC v4 06/11] vduse: Implement an MMU-based IOMMU driver

[Yongji Xie <xieyongji@bytedance.com>]

On Mon, Mar 8, 2021 at 3:04 PM Jason Wang <jasowang@redhat.com> wrote:
>
>
> On 2021/3/8 1:05 下午, Yongji Xie wrote:
> > On Mon, Mar 8, 2021 at 11:52 AM Jason Wang <jasowang@redhat.com> wrote:
> >>
> >> On 2021/3/8 11:45 上午, Yongji Xie wrote:
> >>> On Mon, Mar 8, 2021 at 11:17 AM Jason Wang <jasowang@redhat.com> wrote:
> >>>> On 2021/3/5 3:59 下午, Yongji Xie wrote:
> >>>>> On Fri, Mar 5, 2021 at 3:27 PM Jason Wang <jasowang@redhat.com> wrote:
> >>>>>> On 2021/3/5 3:13 下午, Yongji Xie wrote:
> >>>>>>> On Fri, Mar 5, 2021 at 2:52 PM Jason Wang <jasowang@redhat.com> 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].
> >>
> >> It should still be [0, 64M). Do you see any issue?
> >>
> > Does it mean we still need to map the whole bounce buffer into itree
> > (dev->iommu) at initialization?
>
>
> It's your choice I think, the point is to use a single IOTLB for
> maintaining mappings of all types of pages (bounce, coherent, or shared).
>

OK, got it.

Thanks,
Yongji