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

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Date:   Fri, 5 Mar 2021 15:27:11 +0800
From:   Jason Wang <jasowang@...hat.com>
To:     Yongji Xie <xieyongji@...edance.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: [RFC v4 06/11] vduse: Implement an MMU-based IOMMU driver


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.

In the case of bounce page, it's bascially the whole range of bounce buffer?

Thanks


> 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
>