On 2023-07-21 at 14:26:11 +0800, Yan Zhao wrote:

On Tue, Jul 18, 2023 at 04:44:44PM -0700, Sean Christopherson wrote:

May I know why KVM now needs to register to callback .change_pte()?

I can see the original purpose is to "setting a pte in the shadow page
table directly, instead of flushing the shadow page table entry and then
getting vmexit to set it"[1].

IIUC, KVM is expected to directly make the new pte present for new
pages in this callback, like for COW.

As also commented in kvm_mmu_notifier_change_pte(), .change_pte() must be
surrounded by .invalidate_range_{start,end}().

While kvm_mmu_notifier_invalidate_range_start() has called kvm_unmap_gfn_range()
to zap all leaf SPTEs, and page fault path will not install new SPTEs
successfully before kvm_mmu_notifier_invalidate_range_end(),
kvm_set_spte_gfn() should not be able to find any shadow present leaf entries to
update PFN.

I also failed to figure out how the kvm_set_spte_gfn() could pass
several !is_shadow_present_pte(iter.old_spte) check then write the new
pte.


[1] https://lore.kernel.org/all/200909222039.n8MKd4TL002696@imap1.linux-foundation.org/ (local)

Thanks,
Yilun

Or could we just delete completely
"kvm_handle_hva_range(mn, address, address + 1, pte, kvm_set_spte_gfn);"
from kvm_mmu_notifier_change_pte() ?