Jason Wang
2020-Dec-31 05:49 UTC
[RFC v2 09/13] vduse: Add support for processing vhost iotlb message
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? Thanks> And in streaming DMA cases, there is no limit. So another choice is > using vmalloc'ed memory only for coherent DMA cases. > > Not sure if this is clear for you. > > Thanks, > Yongji >