> From: Peter Xu
> Sent: Friday, February 23, 2018 3:09 PM
> >
> > Right. I think my question was not clear. My question was that why don’t
> > IOMMU invalidate device-iotlb along with its mappings in one go. Then
> > device driver doesn’t need to flush device-iotlb explicitly. Maybe the
> > reason is that ATS and IOMMU are not always coupled.. but I guess it’s
> time
> > for me to get some more background :)
> Ah, I see your point.
> I don't know the answer.  My wild guess is that IOMMU is just trying
> to be simple and only provide most basic functionalities, leaving
> complex stuff to CPU.  For example, if IOMMU takes over the ownership
> to deliever device-iotlb invalidations when receiving iotlb
> invalidations, it possibly needs to traverse the device tree sometimes
> (e.g., for domain invalidations) to know what device is under what
> domain, which is really compliated.  While it'll be simpler for CPU to
> do this since it's very possible that the OS keeps a list of devices
> for a domain already.
> IMHO that follows the *nix philosophy too - Do One Thing And Do It
> Well.  Though again, it's wild guess and I may be wrong. :)
> CCing Alex, in case he has quick answers.

IOMMU and devices are de-coupled. You need a protocol so IOMMU
knows which device enables translation caches and thus requires
explicit invalidation, which is how ATS comes to play. ATS is not
mandatory for vhost, but doing so provides more flexibility e.g.
to enable I/O page fault if further emulating PCI PRS cap.


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