Re: [PATCH v2 1/3] KVM: x86: add a new page track hook track_remove_slot

[Sean Christopherson <seanjc@google.com>]

On Tue, Nov 15, 2022, Yan Zhao wrote:
> On Mon, Nov 14, 2022 at 04:32:34PM +0000, Sean Christopherson wrote:
> > On Mon, Nov 14, 2022, Yan Zhao wrote:
> > > On Sat, Nov 12, 2022 at 12:43:07AM +0000, Sean Christopherson wrote:
> > > > On Sat, Nov 12, 2022, Yan Zhao wrote:
> > > > > And I'm also not sure if a slots_arch_lock is required for
> > > > > kvm_slot_page_track_add_page() and kvm_slot_page_track_remove_page().
> > > > 
> > > > It's not required.  slots_arch_lock protects interaction between memslot updates
> > > In kvm_slot_page_track_add_page() and kvm_slot_page_track_remove_page(),
> > > slot->arch.gfn_track[mode][index] is updated in update_gfn_track(),
> > > do you know which lock is used to protect it?
> > 
> > mmu_lock protects the count, kvm->srcu protects the slot, and shadow_root_allocated
> > protects that validity of gfn_track, i.e. shadow_root_allocated ensures that KVM
> > allocates gfn_track for all memslots when shadow paging is activated.
> Hmm, thanks for the reply.
> but in direct_page_fault(),
> if (page_fault_handle_page_track(vcpu, fault))
> 	return RET_PF_EMULATE;
> 
> slot->arch.gfn_track is read without any mmu_lock is held.

That's a fast path that deliberately reads out of mmu_lock.  A false positive
only results in unnecessary emulation, and any false positive is inherently prone
to races anyways, e.g. fault racing with zap.

> > arch/x86/kvm/mmu/page_track.c-void __kvm_write_track_remove_gfn(struct kvm *kvm,
> > arch/x86/kvm/mmu/page_track.c-                            struct kvm_memory_slot *slot, gfn_t gfn)
> > arch/x86/kvm/mmu/page_track.c-{
> > arch/x86/kvm/mmu/page_track.c-  lockdep_assert_held_write(&kvm->mmu_lock);
> > arch/x86/kvm/mmu/page_track.c-
> > arch/x86/kvm/mmu/page_track.c-  if (KVM_BUG_ON(!kvm_page_track_write_tracking_enabled(kvm), kvm))
> > arch/x86/kvm/mmu/page_track.c-          return;
> > arch/x86/kvm/mmu/page_track.c-
> > arch/x86/kvm/mmu/page_track.c:  update_gfn_write_track(slot, gfn, -1);
> yes, it will be helpful.
> 
> Besides, will WRITE_ONCE or atomic_add in update_gfn_write_track() to
> update slot->arch.gfn_track be better?

WRITE_ONCE() won't suffice, it needs to be atomic.  Switching to atomic_inc/dec
isn't worth it so long as KVM's shadow MMU takes mmu_lock for write, i.e. while
the accounting is mutually exclusive for other reasons in both KVM and KVMGT.