Event channels, yeah. That really is where I started. It was all so simple in Joao and Ankur's original version at https://www.spinics.net/lists/kvm/msg182556.html — just a handful of simple test_and_set_bit() calls on the mapped page. When I posted v1 I didn't quite understand how steal time and nesting were safely using the kvm_map_gfn() function, and I posted the Xen part declaring that I had "reduced it to a previously solved problem". Then I frowned at kvm_map_gfn() for a bit longer, concluded it was basically impossible to use it safely on its own because the page it maps might belong to another guest by the time it even returns to its caller, and posted a v2 in which I did something safer for myself by hooking into the MMU notifiers. I then fixed the steal time reporting, and killed gfn_to_pfn_cache, under separate cover. In v3 of this series I re-introduced a saner gfn_to_pfn_cache with MMU notifier support to give it proper invalidation semantics. This can now be used for the Xen event channel support and should also be usable for fixing the various use-after-free races in the nesting code too — the last patch in this series being an untested proof of concept attempt at fixing one such. Since adding a C file in virt/kvm/ was somewhat more painful than it really should have been, there is a small detour into all the arch specific Makefiles to make them include a common one. v4: Rework the dirty marking given the relevation that it can only be done from the context of an active vCPU. So just defer it to happen in the unmap. Also introduce a lightweight unmap call instead of just the full destroy. Document the Xen shared info page as NOT participating in dirty tracking. Fix a typo in the CONFIG_HAVE_KVM_DIRTY_RING patch which prevented the capability from being advertised. Intended for merging up to patch 10. Patch 11 is for illustration.
