Messages in this thread | ![/](/images/icornerl.gif) | | From | "Tian, Kevin" <> | Subject | RE: [PATCH v2 1/3] docs: IOMMU user API | Date | Wed, 17 Jun 2020 08:28:24 +0000 |
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> From: Liu, Yi L <yi.l.liu@intel.com> > Sent: Wednesday, June 17, 2020 2:20 PM > > > From: Jacob Pan <jacob.jun.pan@linux.intel.com> > > Sent: Tuesday, June 16, 2020 11:22 PM > > > > On Thu, 11 Jun 2020 17:27:27 -0700 > > Jacob Pan <jacob.jun.pan@linux.intel.com> wrote: > > > > > > > > > > But then I thought it even better if VFIO leaves the entire > > > > copy_from_user() to the layer consuming it. > > > > > > > OK. Sounds good, that was what Kevin suggested also. I just wasn't > > > sure how much VFIO wants to inspect, I thought VFIO layer wanted to do > > > a sanity check. > > > > > > Anyway, I will move copy_from_user to iommu uapi layer. > > > > Just one more point brought up by Yi when we discuss this offline. > > > > If we move copy_from_user to iommu uapi layer, then there will be > multiple > > copy_from_user calls for the same data when a VFIO container has > multiple domains, > > devices. For bind, it might be OK. But might be additional overhead for TLB > flush > > request from the guest. > > I think it is the same with bind and TLB flush path. will be multiple > copy_from_user.
multiple copies is possibly fine. In reality we allow only one group per nesting container (as described in patch [03/15]), and usually there is just one SVA-capable device per group.
> > BTW. for moving data copy to iommy layer, there is another point which > need to consider. VFIO needs to do unbind in bind path if bind failed, > so it will assemble unbind_data and pass to iommu layer. If iommu layer > do the copy_from_user, I think it will be failed. any idea? >
This might be mitigated if we go back to use the same bind_data for both bind/unbind. Then you can reuse the user object for unwinding.
However there is another case where VFIO may need to assemble the bind_data itself. When a VM is killed, VFIO needs to walk allocated PASIDs and unbind them one-by-one. In such case copy_from_user doesn't work since the data is created by kernel. Alex, do you have a suggestion how this usage can be supported? e.g. asking IOMMU driver to provide two sets of APIs to handle user/kernel generated requests?
Thanks Kevin
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