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

[James Houghton <jthoughton@google.com>]

On Wed, May 28, 2025 at 11:09 AM James Houghton <jthoughton@google.com> wrote:
>
> On Tue, May 6, 2025 at 8:06 PM Sean Christopherson <seanjc@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.

Ah it wouldn't break that example, as `new` would be NULL. But I think
it would break the case where we are moving a memslot that keeps
`KVM_MEM_LOG_DIRTY_PAGES`.

>
> >
> >         /*
> >          * 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;