Messages in this thread | | | Subject | Re: [PATCH 3/5] vDPA: introduce vDPA bus | From | Jason Wang <> | Date | Tue, 21 Jan 2020 16:35:27 +0800 |
| |
On 2020/1/21 下午4:15, Michael S. Tsirkin wrote: > 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.
Yes, I think that works but need confirmation from Shahaf or Jason.
Thanks
> >>> 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 >>>>
| |