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

[Jason Wang <jasowang@redhat.com>]

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).

Thanks


>
> Thanks,
> Yongji
>