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

[Sean Christopherson <seanjc@google.com>]

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
mmu_first_shadow_root_alloc().  When CONFIG_KVM_EXTERNAL_WRITE_TRACKING=y, then
the mmu_first_shadow_root_alloc() doesn't touch the memslots because everything
is pre-allocated:

bool kvm_page_track_write_tracking_enabled(struct kvm *kvm)
{
	return IS_ENABLED(CONFIG_KVM_EXTERNAL_WRITE_TRACKING) ||
	       !tdp_enabled || kvm_shadow_root_allocated(kvm);
}

int kvm_page_track_create_memslot(struct kvm *kvm,
				  struct kvm_memory_slot *slot,
				  unsigned long npages)
{
	if (!kvm_page_track_write_tracking_enabled(kvm)) <== always true
		return 0;

	return __kvm_page_track_write_tracking_alloc(slot, npages);
}

Though now that you point it out, it's tempting to #ifdef out some of those hooks
so that's basically impossible for mmu_first_shadow_root_alloc() to cause problems.
Not sure the extra #ideffery would be worth while though.

slots_arch_lock also protects shadow_root_allocated, but that's a KVM-internal
detail that isn't relevant to the page-tracking machinery when
CONFIG_KVM_EXTERNAL_WRITE_TRACKING=y.