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

[Shahaf Shuler]

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

> 
> 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
> >>>>