RE: [RFC PATCH v2] uacce: Add uacce_ctrl misc device

[Tian, Kevin]

> From: Jason Gunthorpe <jgg@xxxxxxxx>
> Sent: Tuesday, February 2, 2021 7:44 AM
> 
> On Fri, Jan 29, 2021 at 10:09:03AM +0000, Tian, Kevin wrote:
> > > SVA is not doom to work with IO page fault only. If we have SVA+pin,
> > > we would get both sharing address and stable I/O latency.
> >
> > Isn't it like a traditional MAP_DMA API (imply pinning) plus specifying
> > cpu_va of the memory pool as the iova?
> 
> I think their issue is the HW can't do the cpu_va trick without also
> involving the system IOMMU in a SVA mode
> 

This is the part that I didn't understand. Using cpu_va in a MAP_DMA
interface doesn't require device support. It's just an user-specified
address to be mapped into the IOMMU page table. On the other hand,
sharing CPU page table through a SVA interface for an usage where I/O 
page faults must be completely avoided seems a misleading attempt. 
Even if people do want this model (e.g. mix pinning+fault), it should be
a mm syscall as Greg pointed out, not specific to sva.

Thanks
Kevin