On Tue, Dec 29, 2020 at 5:11 PM Jason Wang [off-list ref] wrote:

----- Original Message -----

quoted

On Mon, Dec 28, 2020 at 4:43 PM Jason Wang [off-list ref] wrote:

quoted

On 2020/12/28 下午4:14, Yongji Xie wrote:

quoted

quoted

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. 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.

Thanks,
Yongji