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.
