Re: [RFC PATCH 0/4] KVM: x86/mmu: Rework marking folios dirty/accessed

Paolo Bonzini
2024-04-05T14:06:38Z

On 4/5/24 15:59, Sean Christopherson wrote:
> Ya, from commit c13fda237f08 ("KVM: Assert that notifier count is elevated in
> .change_pte()"):
>
> x86 and MIPS are clearcut nops if the old SPTE is not-present, and that
> is guaranteed due to the prior invalidation. PPC simply unmaps the SPTE,
> which again should be a nop due to the invalidation. arm64 is a bit
> murky, but it's also likely a nop because kvm_pgtable_stage2_map() is
> called without a cache pointer, which means it will map an entry if and
> only if an existing PTE was found.
>
> I'm 100% in favor of removing .change_pte(). As I've said multiple times, the
> only reason I haven't sent a patch is because I didn't want it to prompt someone
> into resurrecting the original behavior. 🙂

Ah, this indeed reminded me of discussions we had in the past. Patches
sent now:

https://lore.kernel.org/kvm/20240405115815.3226315-1-pbonzini@redhat.com/

and I don't think anyone would try to resurrect the original behavior.
It made some sense when there was an .invalidate_page() callback as
well, but the whole thing started to crumble when .invalidate_page() was
made sleepable, plus as David noticed it's very poorly documented where
to use set_pte_at_notify() vs. set_pte_at().

As I wrote in the cover letter above, the only reason why it has never
caused a bug is because it was doing nothing...

Paolo