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

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Date:   Mon, 8 Mar 2021 11:45:03 +0800
From:   Yongji Xie <xieyongji@...edance.com>
To:     Jason Wang <jasowang@...hat.com>
Cc:     "Michael S. Tsirkin" <mst@...hat.com>,
        Stefan Hajnoczi <stefanha@...hat.com>,
        Stefano Garzarella <sgarzare@...hat.com>,
        Parav Pandit <parav@...dia.com>, Bob Liu <bob.liu@...cle.com>,
        Christoph Hellwig <hch@...radead.org>,
        Randy Dunlap <rdunlap@...radead.org>,
        Matthew Wilcox <willy@...radead.org>, viro@...iv.linux.org.uk,
        Jens Axboe <axboe@...nel.dk>, bcrl@...ck.org,
        Jonathan Corbet <corbet@....net>,
        virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org,
        kvm@...r.kernel.org, linux-aio@...ck.org,
        linux-fsdevel@...r.kernel.org
Subject: Re: Re: [RFC v4 06/11] vduse: Implement an MMU-based IOMMU driver

On Mon, Mar 8, 2021 at 11:17 AM Jason Wang <jasowang@...hat.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@...hat.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@...hat.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].

Thanks,
Yongji