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Date:   Wed, 30 Dec 2020 16:41:30 +0800
From:   Jason Wang <>
To:     Yongji Xie <>
Cc:     "Michael S. Tsirkin" <>,
        Stefan Hajnoczi <>,,
        Parav Pandit <>,,
        Randy Dunlap <>,
        Matthew Wilcox <>,,,,,,,,,,
Subject: Re: [RFC v2 09/13] vduse: Add support for processing vhost iotlb

On 2020/12/30 下午3:09, Yongji Xie wrote:
> On Wed, Dec 30, 2020 at 2:11 PM Jason Wang <> wrote:
>> On 2020/12/29 下午6:26, Yongji Xie wrote:
>>> On Tue, Dec 29, 2020 at 5:11 PM Jason Wang <> wrote:
>>>> ----- Original Message -----
>>>>> On Mon, Dec 28, 2020 at 4:43 PM Jason Wang <> 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().

>>> It's true that userspace can
>>> access the dma buffer if userspace doesn't do the unmap. But the dma
>>> pages would not be freed and reused unless user space called munmap()
>>> for them.
>> I wonder whether or not we could recycle IOVA in this case to avoid the
>> IOTLB_UMAP message.
> We can achieve that if we use vmalloc() to do allocation for the
> bounce buffer which can be used in coherent DMA mapping case. But
> looks like we still have no way to avoid the IOTLB_UMAP message in
> vhost-vdpa case.

I think that's fine. For virtio-vdpa, from VDUSE userspace perspective, 
it works like a driver that is using SWIOTLB in this case.


> Thanks,
> Yongji

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