Jason Wang
2020-Dec-31 02:49 UTC
[RFC v2 09/13] vduse: Add support for processing vhost iotlb message
On 2020/12/30 ??6:12, Yongji Xie wrote:> On Wed, Dec 30, 2020 at 4:41 PM Jason Wang <jasowang at redhat.com> wrote: >> >> On 2020/12/30 ??3:09, Yongji Xie wrote: >>> On Wed, Dec 30, 2020 at 2:11 PM Jason Wang <jasowang at redhat.com> wrote: >>>> On 2020/12/29 ??6:26, Yongji Xie wrote: >>>>> On Tue, Dec 29, 2020 at 5:11 PM Jason Wang <jasowang at redhat.com> wrote: >>>>>> ----- Original Message ----- >>>>>>> On Mon, Dec 28, 2020 at 4:43 PM Jason Wang <jasowang at redhat.com> wrote: >>>>>>>> On 2020/12/28 ??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. 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? Thanks> >>>>> 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. >> > OK, will do it in v3. > > Thanks, > Yongji >