On Thu, 2022-07-28 at 10:46 +0300, Maxim Levitsky wrote: > On Mon, 2022-07-25 at 16:08 +0000, Sean Christopherson wrote: > > 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. > Makes sense. > > > 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. > > The thing is that KVM goes over the list of notifiers and calls them for every write from the emulator > in fact even just for mmio write, and when you enable write tracking on a page, > you just write protect the page and add a mark in the page track array, which is roughly > > 'don't install spte, don't install mmio spte, but just emulate the page fault if it hits this page' > > So adding more than a bare minimum to this list, seems just a bit wrong. > > > > 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. > > The thing is that if I register multiple notifiers, they all will be called anyway, > but yes I can use container_of, and discover which table the notifier belongs to, > instead of having a hash table where I lookup the GFN of the fault. > > The above means practically that all the shadow physid tables will be in a linear > list of notifiers, so I could indeed avoid per vm mutex on the write tracking, > however for simplicity I probably will still need it because I do modify the page, > and having per physid table mutex complicates things. > > Currently in my code the locking is very simple and somewhat dumb, but the performance > is very good because the code isn't executed often, most of the time the AVIC hardware > works alone without any VM exits. > > Once the code is accepted upstream, it's one of the things that can be improved. > > > Note though that I still need a hash table and a mutex because on each VM entry, > the guest can use a different physid table, so I need to lookup it, and create it, > if not found, which would require read/write of the hash table and thus a 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. > > > > True, but then what about adding a refcount to 'struct kvm_page_track_notifier_node' > instead of a boolean, and allowing redundant registration? > Probably not worth it, in which case I am OK to add a refcount to my avic code. > > Or maybe just scrap the whole thing and just leave registration and activation of the > write tracking as two separate things? Honestly now that looks like the most clean > solution. Kind ping on this. Do you still want me to enable write tracking on the notifier registeration, or scrap the idea? Best regards, Maxim Levitsky > > Best regards, > Maxim Levitsky
