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

[Jason Wang]

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


> 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