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Date: Wed, 30 Dec 2020 14:10:57 +0800 From: Jason Wang <jasowang@...hat.com> To: Yongji Xie <xieyongji@...edance.com> Cc: "Michael S. Tsirkin" <mst@...hat.com>, Stefan Hajnoczi <stefanha@...hat.com>, sgarzare@...hat.com, Parav Pandit <parav@...dia.com>, akpm@...ux-foundation.org, Randy Dunlap <rdunlap@...radead.org>, Matthew Wilcox <willy@...radead.org>, viro@...iv.linux.org.uk, axboe@...nel.dk, bcrl@...ck.org, corbet@....net, virtualization@...ts.linux-foundation.org, netdev@...r.kernel.org, kvm@...r.kernel.org, linux-aio@...ck.org, linux-fsdevel@...r.kernel.org, linux-mm@...ck.org Subject: Re: [RFC v2 09/13] vduse: Add support for processing vhost iotlb message On 2020/12/29 下午6:26, Yongji Xie wrote: > On Tue, Dec 29, 2020 at 5:11 PM Jason Wang <jasowang@...hat.com> wrote: >> >> >> ----- Original Message ----- >>> On Mon, Dec 28, 2020 at 4:43 PM Jason Wang <jasowang@...hat.com> 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? > 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. Thanks > > Thanks, > Yongji >
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