Re: [RFC PATCH v3 04/19] KVM: x86: mmu: allow to enable write tracking externally

[Maxim Levitsky <mlevitsk@redhat.com>]

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