On Wed, Jul 20, 2022, Maxim Levitsky wrote: > On Sun, 2022-05-22 at 13:22 +0300, Maxim Levitsky wrote: > > On Thu, 2022-05-19 at 16:37 +0000, Sean Christopherson wrote: > > > On Wed, Apr 27, 2022, Maxim Levitsky wrote: > > > > @@ -5753,6 +5752,10 @@ int kvm_mmu_init_vm(struct kvm *kvm) > Now for nested AVIC, this is what I would like to do: > > - just like mmu, I prefer to register the write tracking notifier, when the > VM is created. > > - just like mmu, write tracking should only be enabled when nested AVIC is > actually used first time, so that write tracking is not always enabled when > you just boot a VM with nested avic supported, since the VM might not use > nested at all. > > Thus I either need to use the __kvm_page_track_register_notifier too for AVIC > (and thus need to export it) or I need to have a boolean > (nested_avic_was_used_once) and register the write tracking notifier only > when false and do it not on VM creation but on first attempt to use nested > AVIC. > > Do you think this is worth it? I mean there is some value of registering the > notifier only when needed (this way it is not called for nothing) but it does > complicate things a bit. Compared to everything else that you're doing in the nested AVIC code, refcounting the shared kvm_page_track_notifier_node object is a trivial amount of complexity. And on that topic, do you have performance numbers to justify using a single shared node? E.g. if every table instance has its own notifier, then no additional refcounting is needed. It's not obvious that a shared node will provide better performance, e.g. if there are only a handful of AVIC tables being shadowed, then a linear walk of all nodes is likely fast enough, and doesn't bring the risk of a write potentially being stalled due to having to acquire a VM-scoped mutex. > I can also stash this boolean (like 'bool registered;') into the 'struct > kvm_page_track_notifier_node', and thus allow the > kvm_page_track_register_notifier to be called more that once - then I can > also get rid of __kvm_page_track_register_notifier. No, allowing redundant registration without proper refcounting leads to pain, e.g. X registers, Y registers, X unregisters, kaboom.
