On Tue, Jan 21, 2020 at 04:00:38PM +0800, Jason Wang wrote: > > On 2020/1/21 下午1:47, Michael S. Tsirkin wrote: > > On Tue, Jan 21, 2020 at 12:00:57PM +0800, Jason Wang wrote: > > > On 2020/1/21 上午1:49, Jason Gunthorpe wrote: > > > > On Mon, Jan 20, 2020 at 04:43:53PM +0800, Jason Wang wrote: > > > > > This is similar to the design of platform IOMMU part of vhost-vdpa. We > > > > > decide to send diffs to platform IOMMU there. If it's ok to do that in > > > > > driver, we can replace set_map with incremental API like map()/unmap(). > > > > > > > > > > Then driver need to maintain rbtree itself. > > > > I think we really need to see two modes, one where there is a fixed > > > > translation without dynamic vIOMMU driven changes and one that > > > > supports vIOMMU. > > > > > > I think in this case, you meant the method proposed by Shahaf that sends > > > diffs of "fixed translation" to device? > > > > > > It would be kind of tricky to deal with the following case for example: > > > > > > old map [4G, 16G) new map [4G, 8G) > > > > > > If we do > > > > > > 1) flush [4G, 16G) > > > 2) add [4G, 8G) > > > > > > There could be a window between 1) and 2). > > > > > > It requires the IOMMU that can do > > > > > > 1) remove [8G, 16G) > > > 2) flush [8G, 16G) > > > 3) change [4G, 8G) > > > > > > .... > > Basically what I had in mind is something like qemu memory api > > > > 0. begin > > 1. remove [8G, 16G) > > 2. add [4G, 8G) > > 3. commit > > > This sounds more flexible e.g driver may choose to implement static mapping > one through commit. But a question here, it looks to me this still requires > the DMA to be synced with at least commit here. Otherwise device may get DMA > fault? Or device is expected to be paused DMA during begin? > > Thanks For example, commit might switch one set of tables for another, without need to pause DMA. > > > > > Anyway, I'm fine with a one-shot API for now, we can > > improve it later. > > > > > > There are different optimization goals in the drivers for these two > > > > configurations. > > > > > > > > > > If the first one, then I think memory hotplug is a heavy flow > > > > > > regardless. Do you think the extra cycles for the tree traverse > > > > > > will be visible in any way? > > > > > I think if the driver can pause the DMA during the time for setting up new > > > > > mapping, it should be fine. > > > > This is very tricky for any driver if the mapping change hits the > > > > virtio rings. :( > > > > > > > > Even a IOMMU using driver is going to have problems with that.. > > > > > > > > Jason > > > > > > Or I wonder whether ATS/PRI can help here. E.g during I/O page fault, > > > driver/device can wait for the new mapping to be set and then replay the > > > DMA. > > > > > > Thanks > > > > >
