[PATCH v2 06/13] KVM: arm64: Add support for KVM_MEM_USERFAULT

[James Houghton]

On Tue, May 6, 2025 at 8:06 PM Sean Christopherson <seanjc at google.com> wrote:
>
> On Thu, Jan 09, 2025, James Houghton wrote:
> > @@ -2073,6 +2080,23 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
> >                                  enum kvm_mr_change change)
> >  {
> >       bool log_dirty_pages = new && new->flags & KVM_MEM_LOG_DIRTY_PAGES;
> > +     u32 new_flags = new ? new->flags : 0;
> > +     u32 changed_flags = (new_flags) ^ (old ? old->flags : 0);
>
> This is a bit hard to read, and there's only one use of log_dirty_pages.  With
> zapping handled in common KVM, just do:

Thanks, Sean. Yeah what you have below looks a lot better, thanks for
applying it for me. I'll post a new version soon. One note below.

>
> @@ -2127,14 +2131,19 @@ void kvm_arch_commit_memory_region(struct kvm *kvm,
>                                    const struct kvm_memory_slot *new,
>                                    enum kvm_mr_change change)
>  {
> -       bool log_dirty_pages = new && new->flags & KVM_MEM_LOG_DIRTY_PAGES;
> +       u32 old_flags = old ? old->flags : 0;
> +       u32 new_flags = new ? new->flags : 0;
> +
> +       /* Nothing to do if not toggling dirty logging. */
> +       if (!((old_flags ^ new_flags) & KVM_MEM_LOG_DIRTY_PAGES))
> +               return;

This is my bug, not yours, but I think this condition must also check
that `change == KVM_MR_FLAGS_ONLY` for it to be correct. This, for
example, will break the case where we are deleting a memslot that
still has KVM_MEM_LOG_DIRTY_PAGES enabled. Will fix in the next
version.

>
>         /*
>          * At this point memslot has been committed and there is an
>          * allocated dirty_bitmap[], dirty pages will be tracked while the
>          * memory slot is write protected.
>          */
> -       if (log_dirty_pages) {
> +       if (new_flags & KVM_MEM_LOG_DIRTY_PAGES) {
>
>                 if (change == KVM_MR_DELETE)
>                         return;