Hi Kevin, On Fri, Feb 23, 2018 at 2:34 AM, Tian, Kevin <kevin.t...@intel.com> wrote: >> 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 >> IOMMU >> > 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.
Thanks for the explanation! Thanks, Jintack > > Thanks > Kevin
