Re: [PATCH RFC v2 00/18] Add VFIO mediated device support and DEV-MSI support for the idxd driver

[Jason Wang]

On 2020/8/12 下午12:05, Tian, Kevin wrote:
> > The problem is that if we tie all controls via VFIO uAPI, the other
> > subsystem like vDPA is likely to duplicate them. I wonder if there is a
> > way to decouple the vSVA out of VFIO uAPI?
> vSVA is a per-device (either pdev or mdev) feature thus naturally should
> be managed by its device driver (VFIO or vDPA). From this angle some
> duplication is inevitable given VFIO and vDPA are orthogonal passthrough
> frameworks. Within the kernel the majority of vSVA handling is done by
> IOMMU and IOASID modules thus most logic are shared.


So why not introduce vSVA uAPI at IOMMU or IOASID layer?


> > >    If an userspace DMA interface can be easily
> > > adapted to be a passthrough one, it might be the choice.
> > It's not that easy even for VFIO which requires a lot of new uAPIs and
> > infrastructures(e.g mdev) to be invented.
> >
> >
> > > But for idxd,
> > > we see mdev a much better fit here, given the big difference between
> > > what userspace DMA requires and what guest driver requires in this hw.
> > A weak point for mdev is that it can't serve kernel subsystem other than
> > VFIO. In this case, you need some other infrastructures (like [1]) to do
> > this.
> mdev is not exclusive from kernel usages. It's perfectly fine for a driver
> to reserve some work queues for host usages, while wrapping others
> into mdevs.


I meant you may want slices to be an independent device from the kernel 
point of view:

E.g for ethernet devices, you may want 10K mdevs to be passed to guest.

Similarly, you may want 10K net devices which is connected to the kernel 
networking subsystems.

In this case it's not simply reserving queues but you need some other 
type of device abstraction. There could be some kind of duplication 
between this and mdev.

Thanks


> Thanks
> Kevin