Re: [RFC v2 09/13] vduse: Add support for processing vhost iotlb message

[Jason Wang <jasowang@redhat.com>]

On 2020/12/31 下午2:52, Yongji Xie wrote:
> On Thu, Dec 31, 2020 at 1:50 PM Jason Wang <jasowang@redhat.com> wrote:
>>
>> On 2020/12/31 下午1:15, Yongji Xie wrote:
>>> On Thu, Dec 31, 2020 at 10:49 AM Jason Wang <jasowang@redhat.com> wrote:
>>>> On 2020/12/30 下午6:12, Yongji Xie wrote:
>>>>> On Wed, Dec 30, 2020 at 4:41 PM Jason Wang <jasowang@redhat.com> wrote:
>>>>>> On 2020/12/30 下午3:09, Yongji Xie wrote:
>>>>>>> On Wed, Dec 30, 2020 at 2:11 PM Jason Wang <jasowang@redhat.com> wrote:
>>>>>>>> On 2020/12/29 下午6:26, Yongji Xie wrote:
>>>>>>>>> On Tue, Dec 29, 2020 at 5:11 PM Jason Wang <jasowang@redhat.com> wrote:
>>>>>>>>>> ----- Original Message -----
>>>>>>>>>>> On Mon, Dec 28, 2020 at 4:43 PM Jason Wang <jasowang@redhat.com> wrote:
>>>>>>>>>>>> On 2020/12/28 下午4:14, Yongji Xie wrote:
>>>>>>>>>>>>>> I see. So all the above two questions are because VHOST_IOTLB_INVALIDATE
>>>>>>>>>>>>>> is expected to be synchronous. This need to be solved by tweaking the
>>>>>>>>>>>>>> current VDUSE API or we can re-visit to go with descriptors relaying
>>>>>>>>>>>>>> first.
>>>>>>>>>>>>>>
>>>>>>>>>>>>> Actually all vdpa related operations are synchronous in current
>>>>>>>>>>>>> implementation. The ops.set_map/dma_map/dma_unmap should not return
>>>>>>>>>>>>> until the VDUSE_UPDATE_IOTLB/VDUSE_INVALIDATE_IOTLB message is replied
>>>>>>>>>>>>> by userspace. Could it solve this problem?
>>>>>>>>>>>>        I was thinking whether or not we need to generate IOTLB_INVALIDATE
>>>>>>>>>>>> message to VDUSE during dma_unmap (vduse_dev_unmap_page).
>>>>>>>>>>>>
>>>>>>>>>>>> If we don't, we're probably fine.
>>>>>>>>>>>>
>>>>>>>>>>> It seems not feasible. This message will be also used in the
>>>>>>>>>>> virtio-vdpa case to notify userspace to unmap some pages during
>>>>>>>>>>> consistent dma unmapping. Maybe we can document it to make sure the
>>>>>>>>>>> users can handle the message correctly.
>>>>>>>>>> Just to make sure I understand your point.
>>>>>>>>>>
>>>>>>>>>> Do you mean you plan to notify the unmap of 1) streaming DMA or 2)
>>>>>>>>>> coherent DMA?
>>>>>>>>>>
>>>>>>>>>> For 1) you probably need a workqueue to do that since dma unmap can
>>>>>>>>>> be done in irq or bh context. And if usrspace does't do the unmap, it
>>>>>>>>>> can still access the bounce buffer (if you don't zap pte)?
>>>>>>>>>>
>>>>>>>>> I plan to do it in the coherent DMA case.
>>>>>>>> Any reason for treating coherent DMA differently?
>>>>>>>>
>>>>>>> Now the memory of the bounce buffer is allocated page by page in the
>>>>>>> page fault handler. So it can't be used in coherent DMA mapping case
>>>>>>> which needs some memory with contiguous virtual addresses. I can use
>>>>>>> vmalloc() to do allocation for the bounce buffer instead. But it might
>>>>>>> cause some memory waste. Any suggestion?
>>>>>> I may miss something. But I don't see a relationship between the
>>>>>> IOTLB_UNMAP and vmalloc().
>>>>>>
>>>>> In the vmalloc() case, the coherent DMA page will be taken from the
>>>>> memory allocated by vmalloc(). So IOTLB_UNMAP is not needed anymore
>>>>> during coherent DMA unmapping because those vmalloc'ed memory which
>>>>> has been mapped into userspace address space during initialization can
>>>>> be reused. And userspace should not unmap the region until we destroy
>>>>> the device.
>>>> Just to make sure I understand. My understanding is that IOTLB_UNMAP is
>>>> only needed when there's a change the mapping from IOVA to page.
>>>>
>>> Yes, that's true.
>>>
>>>> So if we stick to the mapping, e.g during dma_unmap, we just put IOVA to
>>>> free list to be used by the next IOVA allocating. IOTLB_UNMAP could be
>>>> avoided.
>>>>
>>>> So we are not limited by how the pages are actually allocated?
>>>>
>>> In coherent DMA cases, we need to return some memory with contiguous
>>> kernel virtual addresses. That is the reason why we need vmalloc()
>>> here. If we allocate the memory page by page, the corresponding kernel
>>> virtual addresses in a contiguous IOVA range might not be contiguous.
>>
>> Yes, but we can do that as what has been done in the series
>> (alloc_pages_exact()). Or do you mean it would be a little bit hard to
>> recycle IOVA/pages here?
>>
> Yes, it might be hard to reuse the memory. For example, we firstly
> allocate 1 IOVA/page during dma_map, then the IOVA is freed during
> dma_unmap. Actually we can't reuse this single page if we need a
> two-pages area in the next IOVA allocating. So the best way is using
> IOTLB_UNMAP to free this single page during dma_unmap too.
>
> Thanks,
> Yongji


I get you now. Then I agree that let's go with IOTLB_UNMAP.

Thanks


>