Re: [PATCH 3/5] vDPA: introduce vDPA bus

[Jason Wang]

On 2020/1/21 äå4:15, Michael S. Tsirkin wrote:
> On Tue, Jan 21, 2020 at 04:00:38PM +0800, Jason Wang wrote:
> > On 2020/1/21 äå1:47, Michael S. Tsirkin wrote:
> > > On Tue, Jan 21, 2020 at 12:00:57PM +0800, Jason Wang wrote:
> > > > On 2020/1/21 äå1:49, 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