Jason Wang
2020-Dec-31 07:11 UTC
[RFC v2 09/13] vduse: Add support for processing vhost iotlb message
On 2020/12/31 ??2:52, Yongji Xie wrote:> On Thu, Dec 31, 2020 at 1:50 PM Jason Wang <jasowang at redhat.com> wrote: >> >> On 2020/12/31 ??1:15, Yongji Xie wrote: >>> On Thu, Dec 31, 2020 at 10:49 AM Jason Wang <jasowang at redhat.com> wrote: >>>> 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. >>>> >>> 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, > YongjiI get you now. Then I agree that let's go with IOTLB_UNMAP. Thanks>