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

Yan Zhao
2022-11-14T23:09:09Z

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.
>
> The cleanup series I'm prepping adds lockdep assertions for the relevant paths, e.g.
>
> $ git grep -B 8 -E "update_gfn_write_track.*;"
> arch/x86/kvm/mmu/page_track.c-void __kvm_write_track_add_gfn(struct kvm *kvm, struct kvm_memory_slot *slot,
> arch/x86/kvm/mmu/page_track.c- 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);
> --
> 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?

Thanks
Yan