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
