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Date:   Wed, 30 Dec 2020 14:10:57 +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 2020/12/29 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?

> 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.


> Thanks,
> Yongji

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