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

[Jason Wang]

On 2020/1/21 äå7:09, Shahaf Shuler wrote:
> Tuesday, January 21, 2020 10:35 AM, Jason Wang:
> > Subject: Re: [PATCH 3/5] vDPA: introduce vDPA bus
> >
> >
> > 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.
>  From my side, as I wrote, I would like to see the suggested function prototype along w/ the definition of the expectation from driver upon calling those.
> It is not 100% clear to me what should be the outcome of remove/flush/change/commit


Right, I can do this in next version after the discussion is converged.

Thanks


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