On Fri, Oct 21, 2022 at 05:01:32PM +0200, Niklas Schnelle wrote: > On Fri, 2022-10-21 at 10:36 -0300, Jason Gunthorpe wrote: > > On Fri, Oct 21, 2022 at 02:08:02PM +0200, Niklas Schnelle wrote: > > > On Thu, 2022-10-20 at 08:05 -0300, Jason Gunthorpe wrote: > > > > On Thu, Oct 20, 2022 at 10:51:10AM +0200, Niklas Schnelle wrote: > > > > > > > > > Ok that makes sense thanks for the explanation. So yes my assessment is > > > > > still that in this situation the IOTLB flush is architected to return > > > > > an error that we can ignore. Not the most elegant I admit but at least > > > > > it's simple. Alternatively I guess we could use call_rcu() to do the > > > > > zpci_unregister_ioat() but I'm not sure how to then make sure that a > > > > > subsequent zpci_register_ioat() only happens after that without adding > > > > > too much more logic. > > > > > > > > This won't work either as the domain could have been freed before the > > > > call_rcu() happens, the domain needs to be detached synchronously > > > > > > > > Jason > > > > > > Yeah right, that is basically the same issue I was thinking of for a > > > subsequent zpci_register_ioat(). What about the obvious one. Just call > > > synchronize_rcu() before zpci_unregister_ioat()? > > > > Ah, it can be done, but be prepared to wait >> 1s for synchronize_rcu > > to complete in some cases. > > > > What you have seems like it could be OK, just deal with the ugly racy > > failure > > > > Jason > > I'd tend to go with synchronize_rcu(). It won't leave us with spurious > error logs for the failed IOTLB flushes and as you said one expects > detach to be synchronous. I don't think waiting in it will be a > problem. But this is definitely something you're more of an expert on > so I'll trust your judgement. Looking at other callers of > synchronize_rcu() quite a few of them look to be in similar > detach/release kind of situations though not sure how frequent and > performance critical IOMMU domain detaching is in comparison. I would not do it on domain detaching, that is something triggered by userspace through VFIO and it could theoritically happen alot, eg in vIOMMU scenarios. Jason
