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

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Date:   Fri, 5 Mar 2021 15:13:27 +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 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. 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