On Tue, 2020-12-01 at 16:40 -0800, Ankur Arora wrote: > > How come we get to pin the page and directly dereference it every time, > > while kvm_setup_pvclock_page() has to use kvm_write_guest_cached() > > instead? > > So looking at my WIP trees from the time, this is something that > we went back and forth on as well with using just a pinned page or a > persistent kvm_vcpu_map(). I have a new plan. Screw pinning it and mapping it into kernel space just because we need to do atomic operations on it. Futexes can do atomic operations on userspace; so can we. We just add atomic_cmpxchg_user() and use that. We can add kvm_cmpxchg_guest_offset_cached() and use that from places like record_steal_time(). That works nicely for all of the Xen support needs too, I think — since test-and-set and test-and-clear bit operations all want to be built on cmpxchg. The one thing I can think off offhand that it doesn't address easily is the testing of vcpu_info->evtchn_upcall_pending in kvm_xen_cpu_has_interrupt(), which really does want to be fast so I'm not sure I want to be using copy_from_user() for that. But I think I can open-code that to do the cache checking that kvm_read_guest_offset_cached() does, and use __get_user() directly in the fast path, falling back to actually calling kvm_read_guest_offset_cached() when it needs to. That will suffice until/unless we grow more use cases which would make it worth providing that as a kvm_get_guest...() set of functions for generic use.
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