On Thu, Dec 31, 2020 at 1:50 PM Jason Wang <jasowang@xxxxxxxxxx> wrote: > > > On 2020/12/31 下午1:15, Yongji Xie wrote: > > On Thu, Dec 31, 2020 at 10:49 AM Jason Wang <jasowang@xxxxxxxxxx> wrote: > >> > >> On 2020/12/30 下午6:12, Yongji Xie wrote: > >>> On Wed, Dec 30, 2020 at 4:41 PM Jason Wang <jasowang@xxxxxxxxxx> wrote: > >>>> On 2020/12/30 下午3:09, Yongji Xie wrote: > >>>>> On Wed, Dec 30, 2020 at 2:11 PM Jason Wang <jasowang@xxxxxxxxxx> wrote: > >>>>>> On 2020/12/29 下午6:26, Yongji Xie wrote: > >>>>>>> On Tue, Dec 29, 2020 at 5:11 PM Jason Wang <jasowang@xxxxxxxxxx> wrote: > >>>>>>>> ----- Original Message ----- > >>>>>>>>> On Mon, Dec 28, 2020 at 4:43 PM Jason Wang <jasowang@xxxxxxxxxx> 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
